I and a collegue recently found two concealed camera's in a disused office area. One was running off a plugpack (camera in a ring binder on a bookshelf with other ring binders), one powered by a 9v battery (flat when I found it), concealed in a (dummy, but if they had been smarter) data outlet. The battery powered one had been there at least a year and probably had been dead for months when I removed it. (Going by the date code on the 9v battery).
Both were wireless. 2.4Ghz.(I found the devices in an online electronics catalogue, so I know their specs) You could in theory detect their operation with a RF signal meter. Filtering out bluetooth phones and 802.11 wireless laptops could be challenge though.