W. Leong wrote:

One time I came home from vacation and found lots of dead flies on the
floor.
It was gross. Rusty was boarded at the vet, so he didn't get to eat them.
My windows are all closed so I don't know how they got in.

Probably there were fly eggs someplace from earlier, and they hatched
while you were gone. A disgusting thought, I know, but probable.

Joyce