The bug transmitted on an RF frequency. Bell had mentioned to friends that he believed that he had been bugged, but lacking an RF frequency analyzer, he had been unable to find the bug. He apparently was concerned that both his house and car were bugged. Although why he didn't hire the services of a "bug-sweeper" is beyond me.
Didn't Bell use a field strenth meter (output to vibrator in pocket) to direction-find the spook in the Libertarian Party meetings?
I was wondering that myself.
Perhaps the issue was the quality of RF field strength meter. It may've been that the bug would detect motor-running condition, and broadcast for n minutes after motor-off condition, burst mode or other. Alternators produce a considerable RF field themselves, stereo shops have a brisk business in selling RF suppressors to do-it-yourselfers who upgrade their automobile's stereo. my .02ยข to the philosophical puzzle,,,