Could very well not tx until it receives a wake up message.

True. But you could transmit a variety of RF frequencies at the device and try to detect resonance in the antenna. Unfortunately WiFi uses direct conversion rather than IF demodulation, but there is no reason to assume a covert transmitter would use wifi, which is easily spotted by off the shelf equipment. https://youtu.be/ZxyU_1xUOWc

Micropatch antennas are super hard to detect, even with x-ray, and newer types even harder. https://www.nature.com/articles/s41467-017-00343-8