The more interesting question is, “What is the increase in the risk that information relevant to the warrant which granted the wiretap will now be missed?” This requires a lot more data and assumpions than I’m going to get into at this time.

Also, I still want to know why, if the target doesn’t believe their line might be bugged, we would expect them to be mitigating the risk?

From a pure Risk Management perspective, implementing a countermeasure if you don’t believe there is a threat is irrational behavior since the assessed risk would be zero.

A good point, but what about the case where the target doesn’t believe the line is bugged, but it actually is (presumably the case with ALL successful wiretaps)? Since it is a simple protection mechanism for targets, there is no reason not to send the signal during all conversations.

I’d say the risk is the same in your case because if a wiretap target believes their line is bugged, they will avoid the wiretap risk, not mitigate it by exploiting vulnerabilities in the wiretap system.