From personal experience...
At a prior home, we built a backyard pond (about 10k gals); the
Bullfrogs showed up all by themselves. Kind of interesting since the
only consistent water supplies were an overflow drainage pond, about 1/2
mile away and the Delaware River, about a mile away...those frogs had to
do some major hopping across several roads to get to my backyard. When
we asked the pond guy, he said that's the way it is, the frogs will come
naturally and stay in a place that suits them. He also said that the
frogs could have come from other backyard ponds.
So, I suppose your frogs could have just hopped away to a neighbor's
pond or other natural water source.
She mentioned they were tadpoles and just starting to get there legs...