They are good climbers but they have 2 flaws that can help tracking them if they have been in the house for a long time. 1 they pee all the time, and their bellies get really dirty/greasy so they tend to leave distinctive trails if they have climbed things regularly enough. Kinda like a yellow brown smear. They are really good at squeezing through small gaps so even things you think are ok are not. If you have access to a mid 60's style black light or UV light their urine fluoresces yello...
crap..... yellow and their tears/eye fluid is magenta. Try removing the wood at the bottom of your kitchen units they should just pop off any modern ones. You are looking for droppings that look like chocolate rice as evidence they are around.
Right getting rid of them, you could try the ultrasonic thing, you could put down rat poison, however this leads to dead and decomposing mice and rats wherever they die, which could be somewhere unfortunate like under floorboards. Smelly!
Alternatively there are traps, either the traditional spring traps, or you could try something in the humane spectrum. Just remember that cheese is no good as bait, try peanut butter.