A few years back at one of the PB Cons, there was a presentation about mountain lions and how far they ranged.
It made it clear, to me at least, that a sighting was usually for an animal that was essentially passing through an area vs. actually living in that area, hence the relatively low number of sightings.
While I think this is true in some instances. I just wish science would acknowledge this as a possibility rather than just stand proudly back and say "See, told you there were no mountain lions living here...that cat was a transient from someplace else"
As far as I have noticed in most cases, the witness wasn't claiming the out of place animal was a resident, only that they had experienced a sighting. Too many times science gets caught up on explaining why a population of animals can't be living in an area, rather than just recognizing that a sighting has occurred. I feel that most times it doesn't matter to the witness that a breeding population hasn't yet established itself, only that someone in authority recognizes that they did indeed see something.