Page 106 of the fourth printing of Strategic Operations: Advanced Aerospace Rules says that units with an Active Probe apply a -2 modifier to Detection Checks. Page 204 of the sixth printing of TechManual says that DropShips may mount Active Probes (though since it doesn't include advanced rules, it goes on to specify that they only affect units on ground maps).
Large Craft normally cannot benefit from electronic warfare equipment intended for lighter units: indeed, the ECCM rules for Active Probes on page 99 of SO:AAR cannot be used by DropShips. This leads me to suspect this is an oversight rather than a deliberate inclusion, not least because that is a remarkably powerful effect for a mere 500 kilograms on a DropShip.
If it is, in fact, intended, I have a follow-up question: do the Detection Check modifiers for mounting an Active Probe and a Naval Comm-Scanner Suite stack? This applies to Small Craft as well.
