Out of curiosity I just spent 5 minutes crawling around the back of my Camry. I have no box or little round thing attached to my rear window or anywhere on the rear deck. In fact I still can't find any evidence of any device that is mounted anywhere in the open, and I'm still sticking to my feeling that my system uses the factory beeper. Maybe one day I'll set it off on steady tone and have a better listen to try to track down the original of the sound.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.77L/100km (34.8 mpg) over 37750km (23450mi)
75% city - Ave trip 20 min - 2 Winters