Very interesting and I never heard of it before.
In thinking about it, I can see how it could happen.
A tree frog could very easily come down the vent and into the gray tank. It would be very rare, but could happen.
I wouldn't worry about it happening again. Once you flush several times they will be all gone.
Sounds like you don't like them, but it is very interesting to gather some of their egg mass and put them in a small aquarium, watch them hatch and then form into small frogs.
Also they would do more good then harm, since they would eat any mosquito larvae, or other small bad aquatic critters.
Jack L