Proposition for Modified Double-Blind Sensors (TL / DR summary at the bottom)
As it stands under the current system, most of the sensor systems are just plain worthless. Radar is limited by line-of-sight, woods, buildings, and just about everything else that can possibly be on the board except a clear hex. To top it all off, radar has the shortest range of all the basic sensors except seismic.
Infrared is next on the list. It's limited by all the same things as radar, but has the added handicap of not being able to detect anything that isn't overheating. With double heat sinks everywhere and half the canon designs having 100% heat efficiency, the odds of IR being more useful to you than another sensor are slim. An enemy 'mech can fire three heavy PPC's at you and still not show up on IR if it has enough heat sinks.
Seismic sensors are a joke. With a range of 2 hexes per range bracket, you're better off using some other system (read: magscan) and just counting on rolling short range than you do hoping your target has actually moved AND happens to be within the 2-hex range you rolled.
Which finally brings us to Magscan, the most universally useful of the basic sensors. It penetrates hills, buildings, woods, is not limited by the target's heat or movement. It's even tied with IR for the longest range of the basic sensors. On top of that, it is completely unaffected by stealth systems or ECM of any kind, making it more useful than even the active probes in such cases. There is never a reason to use anything BUT magscan, making the other sensors frustrating and superfluous.
Alternate Rules
I'd like to offer the following as alternate rules for the sensor systems in order to make them all useful in different situations. I haven't provided any hard numbers as I've never been good at game balance.
Since we have four different sensor systems, I've split them up into LOS/Non-LOS types, and into "Detailed ID"/"Unit Detected" types. Infrared and radar are the two line-of-sight sensors, while magscan and seismic are the non-LOS versions. The line-of-sight versions have the longer ranges. Radar and Magscan comprise the "Detailed ID" half of the sensors. When they detect a unit, they can see enough of the profile of the target to tell you whether the target is a 'mech, vehicle, or aerospace fighter, as well as the chassis and model of the detected unit. Seismic and IR will not tell you what type of unit you've detected, only that there is SOMETHING there.
IR being limited to detecting an overheating unit seemed silly to me. The previously mentioned 'mech firing three heavy PPC's at you is generating 45 heat. The fact that it isn't overheating right now is because it's heat sinks have dumped all that heat out into the surrounding environment, right out in the open for all to see. I guarantee, those heat sinks are glowing like cherries. So, the range at which IR can detect a unit is based on the amount of heat dissipated by the unit the previous turn. A walking or running 'mech that hasn't done anything else will be hard to spot. A jumping 'mech will be easier. A 'mech firing large lasers is ridiculously easy to detect. (Not related to double-blind, but I think the same rules should apply to heat-seaking missiles.) If you really want to avoid detection by IR, you can shut down your heat sinks and refuse to vent any of it. Think of the stealth systems on the Normandy in Mass Effect. If you don't vent anything, you can't be spotted, but keeping it bottled up will start to have problems when you hit shutdown and ammo explosion levels.
Radar has longest range of all the basic sensors, out to the same 60 hex range that visual spotting gets in broad daylight. It's still limited by line-of-sight, of course, by buildings in it's way, and is still not guaranteed to detect everything. Additionally, using it means that you're lighting yourself up for every enemy unit out there. Regardless of the sensor they're using at the moment, any enemy with LOS to the radar 'mech gets a "unit detected" hit against it.
Magscan is still a very useful system, though not the absolute pinnacle of sensor technology that it was before. It detects through hills and provides an ID of the unit scanned. It's range is shorter than either of the LOS systems. The range at which magscan will detect a unit is based on the weight of the target. A penalty for light units, and a bonus for heavy and assault units. If you don't care about adding some complexity to the calculations (or if these rules get incorporated into MegaMek), magscan receives a penalty for detecting exotic materials like endosteel and ferro-fibrous armor, and a bonus for picking up the larger containment bottles of XL engines. If the powers-that-be decide, it could end up being EASIER to pick up endosteel and ferro-fibrous under magscan.
Seismic sensors have significantly longer ranges than they do under the canon rules, though still not as much as LOS systems. The range brackets are modified by both the mass and MP expended by the target that turn. A light 'mech running 14 hexes is just as easily detected as an assault walking 3. To use seismic sensors, though, involves a penalty if the detecting unit itself has moved. Gotta keep those feet on the ground if you want to listen.
ECM, during double-blind, would act to fuzz out the images returned by radar and magscan. If a unit is inside a friendly ECM bubble, any hits by radar or magscan are reduced to "unit detected" rather than detailed ID's. If a unit is inside a hostile ECM bubble, the ECM suite can start acting more directly, providing a penalty to all sensor scans by that unit and rendering them all "unit detected" hits.
The effects of stealth systems vary by sensor type, as they do now. Radar and magscan receive penalties to range, as well as only getting a "unit detected" hit if they do manage to tag the stealth system. IR is unaffected as, once the heat is out in the open, it's out. Seismic sensors are unaffected by stealth systems either, though I could easily see a penalty if the target is using a quad 'mech (Weight is being split between four legs, not two) or AES.
As for the active probes... It may contradict canon, but I'd rather see them provide bonuses to the existing systems than be entirely new sensor packages of their own. They'd essentially become advanced data-crunching computers for the standard sensors. If not, I'm not entirely sure how to handle them.
TL / DR Summary:
Magscan is the only useful basic sensor under official rules.
Alternate Rules:
IR detects units based on heat dissipated rather than internal temp. Long range. Only shows a unit in the hex, not which type. LOS only.
Radar has longest range. Reveals the radar-using 'mech to the enemy as a "unit detected". Provides detailed scan of target. LOS only.
Seismic has medium range. Distance modified by target mass * distance traveled. Only shows "unit detected", not type. Non-LOS sensor.
Magscan has medium range. Distance modified by metal content (mass) of target. Shows detailed scan. Non-LOS. Bonus to detect XL engine.
Friendly ECM on a unit turns detailed scans by the enemy into unit detected. Hostile ECM on a scanning 'mech applies penalty to all sensors.
Stealth reduces range and blocks detail scans for radar and magscan. No effect on IR or seismic.
Active probes apply bonuses to basic sensors, are not independent sensors.
Comments and criticisms? Is it too complicated for tabletop play? (Double-blind is already complicated enough in tabletop) Are the proposed rules inherently unbalanced no matter what numbers you plug in for ranges and penalties?
*EDIT* Added TL / DR summary