Dave was the ringleader in this and Nibbles has just learnt from him. What is interesting is that Dave is taking it a step further as when you open his cage in the morning he will now trot off and sit by the door waiting to be let out, and also he is starting to show interest in the cat flap so I don't think it will be long before he gets the hang of it - I think they play dumb just to make us feel superior when in fact they are quite bright!
Of course, our third piggie, Pickles, refuses to come in even when the others have gone and needs to be chased around the garden for 10 minutes until she can be cornered behind a flower pot!