My theory is that during Stage performance, at the end some kind of incident happens, with what is believed to be screaming. The curtains close and the camera drops to the floor. Since Phil said he was only going to expand on the mysteries of fnaf 1 and not take away from them, the bite of 87 was definitely going to be implemented into +'s story in some way.

I don't think it's a stretch to say that foxy bit someone, for the same reasons suggested in the original series. If the individual did die, unlike in fnaf 1, they could be a possible contender for who possesses foxy, as the earliest footage shown in night mode is from the stage performance tape, something the person would've witnessed. It also explains why there isn't a body inside of foxy.