Re: Interesting concept...
"If there's a non-disclosure agreement and the ACLU has no proof they are using this"
LEOs in the USA have tried to claim that NDAs prevent them disclosing how the devices work even when ordered to by the judge during a trial. (NDAs do not trump a court and anyone who refuses to fess up when ordered to by a judge can be tossed in lockup (until they agree to explain) if the judge is sufficiently peeved.
At least one LEO has been sanctioned for this and another came perilously close to being jailed for contempt late last year - in the latter case the entire body of evidence relating to the Stingray intercepts was withdrawn from the prosecution case with the judge's assent.
In other cases, the prosecution has withdrawn cases entirely rather than face being compelled to explain how the devices work.
Judges are also complaining that they're frequently being misled about the use of Stingrays on interception warrants they're signing off on. The requests frequently are made for warrants to use pen records (these are the telco's call records) but Stingrays are the device actually used to gather information.
Judges in several states are so concerned that they are specifically asking if the devices are being used or putting language in warrants prohibiting them.
When the prosecution will drop a case rather than expose how they're gathering evidence there's a strong suspicion that the actual methods used are criminal or involve illicit use of cryptography breaches. (I strongly doubt the claims that Stingrays are unencrypted as that only applies to 2G connections, 3G and 4G are never unencrypted)
There's been some claims made that they don't want to explain how their directionfinding works but phase-driven RDF techniques have been around for decades so that doesn't fit the persistent refusal to have the information come out in court (open or under seal).