There will always be grey areas, but I think the recording/observing equipment would need to be both common and not deployed for the purpose of gathering evidence. If some evidence was somehow in the SSID of a Wifi network, and a police officer happened to see it on a laptop, I wouldn't call that a search (although I also probably wouldn't really call it credible enough evidence to justify any further action). If some conversations were broadcast over FM radio, and a police officer happened to hear it while driving by the source, I wouldn't call that a search.