How can I locate/detect a battery powered, PIR activated digital camera hidden outdoors?

No, not a poacher. It does not transmit a signal but stores images on internal drive. No flash or IR.
Does a PIR have any kind of signature?

You say it is activated by a passive sensor, but once it is activated, does it produce light for night use? if so, wandering around at night, with night vision goggles would probably find it. But if the thing is truly passive it could be difficult. As someone else sort of said, if it is transmitting the pictures via radio, there are detectors available. But if it just stores the pictured for later retrieval that may not work, finding a transmitter is one thing, but detecting the faint stray signals that any electronics make would require very sensitive equipment, still might be possible, but probably a whole lot more difficult than on TV shows.

I would probably be possible, but maybe time consuming, to train a dog to find cameras (by scent)

Why are you looking for the camera? did you forget where you put one that belongs to you? if so you must have at least some idea where it is, just look carefully, if the camera was made for wildlife monitoring it probably wont be too well hidden from someone who knows what they are looking for.
If you are a poacher, you will have the right to examine the evidence against you, so at least after the fact you can learn where the camera was, probably not really what you want, but I'm just saying..

Passive Infrared means you would not be able to see it with an infrared camera. . . unless you might detect the heat of the thing itself?

You might look for wires, or an antenna.

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