I remember years ago I had a Walmart crab in my tank. I would look for him for hours and not find him. Then I'd open the filter and jump when he skittered away there inside the filter.
The best though I was watching a movie, no lights on, and I see this giant spider walking across the carpet. Quick grab a towel to squish it, and when I hit the lights... yup, just the crab.
Moral of the story...seal your lid up tight.