Detecting the flash that is not seen by the human eye or too fast to detect.
A simple detector can be built to pick up the IR flash and lock onto the flash. This is a simple circuit that can be built for about 40 bucks. All you would have to do then is walk around at night and trigger the camera. This will not work during the daytime since the camera will not use flash.
Trail camera sensor (PIR)
The PIR is a detection sensor not an emitter. Since it only detects there is nothing given off by the camera.
Pass thru sensors (Break Beam Sensor)
The trail camera may be using a pass thru sensor which is unlikely but could be rigged to use. This would have an emitter and a detector. This could be of a various number of emitters. Laser, IR.