The rats had escaped!
Strange though, there was no sign of tampering, or a way out of the cage, but still 6 of the 9 rats were running free around the lounge, and from the amount they drank when they were put back they were getting a bit dehydrated.
All is well now mind, and I should really get on with something more productive. After all, that was hours ago and apart from putting in a bit more CS time, and sorting out the paperwork for car tax, I've not done anything else productive...