Any Good Ideas On How To Know Which 'FRENEMY" Is Stealing Goo In Our Town?

I have a question. How do you know someone has clicked an already visited building? I before I read this forum I clicked on houses that looked haunted and it said haunted by ghosts.... does that mean someone already visited that building? I have one other question. Placing gremlins in friends towns when I do then i go back to my town and a thing pops up that says your gremlins have collected so many goo from other towns... ok so does this mean I shouldnt kill gremlins people place in my town so they can collect more goo for my neighbors?