Well, they aren't swimming up through the plumbing. Truly, they aren't!

They CAN crawl through amazingly tiny crevices though. Carefully inspect your house [including closets] any place that has a right angle -where floor meets wall, or wall meets wall, as well as where pipes or wires come in from outside. Kitchen cabinets are often installed over sloppy flooring that has gaps between the floor and the wall.

If you are "lucky", you will see a damp area or feel a cool spot, which indicates a crack or crevice when moisture can seep through, and if moisture can, there is a possibility that a froglet can come through. Track down the source of the dampness and repair. Through-wall areas (where electricity/water comes in or sewer goes out) are notorious for letting critters inside; most can be critter-proofed with steel wool covered with plaster of paris or concrete patching.

If you have a crawl space under the house, block the air vents and then [at night] turn a bright light on in the crawlspace and then walk the outside perimeter -- you will see light coming through anyplace that has a crack or opening. And while you are down there, use steel wool to tightly pack every opening that has a pipe going through. Basements have to be checked like the upstairs: inch by inch and everywhere. Check every place that a pipe goes through a wall, floor, or ceiling; the hole around the pipe can be packed with fine steel wool.

The bad part is that this sort of checking is time-consuming and hard on the knees; the good part is that you will not only fix any potential leaks, you will also have rodent-proofed your house. Believe me, speed of awakening by finding a frog on the toilet seat can be equalled when stepping on a field mouse in the dark.

Oh, and it may or may not work on frogs: buy some original Pine-sol, mix into water (1 quart pine-sol to 3 quarts water) and pour the solution around the perimeter of the house. Humans can't seem to smell it after an hour or so, but the scent is usually sufficient to deter rodents and [for the same reason] may help deter any more frogs from entering.