So, we don't have a cat flap but we put our cat's food on the windowsill of a room that we're in a lot and open the window when he wants to come in and out.
We've occasionally had neighbours' cats chancing their arm and coming in for a bit of food. But for the last few weeks we've had one cat coming in pretty much whenever we open the window (our cat deals with this by turning his back on him. Highly effective. Not).
I'm worried he's a stray and genuinely hungry. DH, who is much more of a cat person than me, says cats are thieves and the reason this cat bolts so much food is because that's how they behave when they're stealing. Is that right do you think? DS's gf is sure this cat belongs to someone nearby. Intruder cat has just devoured a sachet of cat food and some biscuits and is now looking at me hopefully!