If there is pee, there will also be poo. Have a gander and see if you see any droppings. They’re like little black pellets the shape of zeppelins (if you’ve ever had a hamster or gerbil, they’re similar), if you can’t find any I’d be very dubious about mice. In my experience from having them, you hear them scuttling at night. If you’re worried, you can get humane traps for pretty cheap - definitely cheaper than an exterminator.