The frogs have probably hopped it.
It is nice when you have frogs in your pond, but I have never successfully got them to sign a tenancy agreement, so they are free to stay and free to go.
Despite what most people think, frogs don't reside in ponds, they are only found in ponds during mating season, or for the occasional dip. They prefer to reside in long grass / under things (upturned plant pot saucer, big stone etc.) They do like it damp. Although they do travel looking for food, and if they find somewhere more suitable, they will stay there.

The best time to look for them is dusk. If it helps, I have newts this and last year, no frogs. (Yet)