According to the system's writeup on Tactical Operations page 349, specifically the third bullet point, a Void-Signature System is disabled by the loss of any of its critical slots or the loss of all active probes mounted on the unit carrying it.
The problem I'm having here is that the construction rules for the system (on the preceding page) don't actually require any active probes to be installed on a void-sig equipped unit. They do, however, require the system to "be combined with one or more ECM suites" -- so, should the rules text in question refer to those instead?