Bloody cat. The end of her elasticised braided polyester collar was so frayed by her scratching at it that it pulled away from the buckle. The whole thing fell off while she was inside, so I was able to locate the ID tag & the hanging magnet that activates the catch in the cat-door to the outside.
Six months ago I bought a flea collar on Amazon (about 50% off normal retail price) and it was still sealed in its plastic wrapping & storage tin.
So now her magnet & ID tag are attached to her new flea collar. It provides 8 months of flea control.