Kardashian Mansion -- Cops Investigating Pap Trespasser

Looks like Kim Kardashian wasn't lying about a photog trespassing onto Kardashian family territory this week -- we found out ... 1 cop car rolled up to their Calabasas mansion Wednesday ... and we're told police were nosing around about the incident.

As we reported, on Monday Kim posted video online, showing her mother Kris Jenner racing into her backyard in hot pursuit of a paparazzo who had trespassed on her grounds.

It smelled like BS at the time -- after all, why would Kris put herself in that kind of danger??? But sources close to the Kardashians say police already have a solid lead on a suspect, and dropped by Kris' house to speak with her about the incident.