I have a couple of these discs that were part of a promo for Goldschlager liqueur.
. It sounds like the Pet Strobe that JudiW described.
The disc is 2" in diameter, has a clip on the back, and with a press of a small button, it pulses out a flash of red light. It has worked great attached to the top of the dog's collar when out on a night hike.
Manik - Are you in the Yorkshire Dales?
kathy