Central locking / alarm fault

I've got an intermitant fault on a 10-plate Scout. Normally, when locking (single press of remote), the car chirps and lights flash. Recently it sometimes (and I haven't tied down the exact circumstances) locks, flashes, but doesn't chirp. A second press of the remote makes the locks whir (presumably the deadlock un-doing), lights flash and it allways chirps.

Any thoughts?

Its under waranty, but might help to know what we're looking at!

Thanks.