Two mysteries

The house we moved into 5 years ago had a small plastic pond in the backyard. We resurrected it and in the second year there suddenly appeared a frog. It wasn't the usual bullfrog but a nice green yellow one with spots and it was not very big. I think it was a leopard frog. Now it's happened again! I heard a glibpppp and saw a flash but I thought it may have been one of the fish splashing. But I definitely heard and saw the blur in my peripheral vision. Still haven't made a definitive sighting but there was the tell tale wet spot on the rock it had been sitting on.

1) The mystery is how the heck could a frog - two frogs appear from nowhere? We don't have a large body of water anywhere close by just a cache basin but that's about a hundred feet away, about 10-15 feet below the pond and usually doesn't contain water. My wife thinks a storm dropped it in the pond. But wouldn't we see more frogs then? We also have a much larger pond but no frogs there.

2) I've always wondered what the wheeet, wheeet sounds were, coming from the wooded areas during summer in the evenings. Are these sounds made by birds, frogs, insects? I thought it may be the sounds made by the nightjar bird but I have yet to spot one in the early evening hours when there's still light. Anyone have some idea of what make the sounds? It's quite pleasant and would lull you to sleep if you were camping under the trees.