I actually have a fridge alarm which beeps when the door has been opened for a certain amount of time. It's very basic and in the shape of an apple. I think it senses light and beeps once it has been exposed to light for about a minute. I just leave it inside the fridge. I think I bought it from Lakelands a few years ago. The only thing is it tends to beep too soon when I haven't finished rummaging in the fridge but all I do is shut the door and re-open it to give me a bit more time.