It sounds like a really, really good place to put the massive NSA data collection to some good useful purpose for the citizens.
There might be some justification, after all, if we can get rid of the robocallers.
If a device is used on the airwaves in the cell phone bands to emulate a tower, then necessarily, it will have to have a transmitter. Is the device type registered by the FCC, does each emulator have a site license? Does each operator have a license to operate the device?
If it is a "cell phone test device" then it must be associated with a properly licensed technician.
The legal requirements to simply operate the device include much more than the rights of the person of interest. For that reason alone, the concealment of the use of the device would be reason enough to throw out any information obtained from it, even before any case law is considered.
IANAL, but I have had 6 different FCC licenses, and have had to jump through many hoops. (I think only 3 are current now).
The one day you'd sell your soul for something, souls are a glut.