How do fly's?

Question

seem to know when you open the door and come inside. its like they wait by the door and then fly straight in.

and no no lights are on , daytime

Answer 1

Flies have compound eyes(everything is magnified), keen sense of sight and smell. That is why they have to be swatted from above. Try swatting one from the side and see how fast it escapes.
They like warm, stench of rotten anything, and are attracted by even a minuscule amount of light.

Answer 2

they have a keen sense of smell, so when they smell humans, they quickly follow them because they know that humans always have delicious food laying around.they tell their other fly friends that hang around the trash bins to pay attention to the humanoid, tag along on their hair because they can't see you when you're sitting up there and they'll lead you directly to the food sources behind the magical door.