Cat scratching and sniffing catflap?

I've got a cat like that and she's nearly three years old and the cat flap has always been there. She prefers coming in through a quite high up window, than use the cat flap. However since the weather turned cold and the window has been shut I have noticed that she has been using the flap. Not when I might see her though! I put her in the porch last thing at night and first thing in the morning she is outside in the garden wanting to come in. I therefore think that if you ignore your cat and if it thinks you are not looking, he will use the cat flap. For god's sake don't 'catch' him at it, think of his pride.