Away for a week on vacation - now we're seeing strange flies?

Hi all,

Wife and I were out of town for 7 days last week. Since we've returned, we've noticed about 15-20 larger house flies in our first floor, master bathroom. More specifically, they appeared to be coming and going from a piece of floor molding that was slightly pulled away from the tile flooring. The strangest thing about the flies is that they're incredible slow, to the point that I can pick them up in my hands and they do not fly away. I'm wondering if this means the flies have been free to roam in the house, and have gorged themselves to the point of being immobile or something?

Anyway, I got under the house into the crawl space today to look for any immediate explanations, but I'm seeing nothing. I also popped the access panel open for the plumbing (from inside the house), nothing in there either.

Any thoughts? We've lived in the house about 3 months now, and have NEVER seen this type (or presence) of fly. I have to assume it has something to do with the lower activity in the house last week, but I'm only guessing.

Thanks-