Do you have a cover on the toilet lid, or is it just the hard surface? If it’s something soft, cats are notorious for looking for soft surfaces, and elevated, makes it even better. Is the cat there while you are in the bathroom? My cats come in the bathroom, when I’m getting ready for work etc.. I could see them staying, after I left, if they were settled in a soft, elevated place. Also, you stated you have more than one cat. Sometimes one will stake out a place, as THEIRS. . .as in, ” this is MY place. . . there’s one way in, I’ll see you coming, so don’t even think about it.” Is the bathroom warm, or the toilet near a heating vent or other source of heat, or in the sunlight from a window? Those will entice a cat . After I turn the heat up in the morning, one of my cats curls up next to the floor vent, so it blows on her. (Sometimes I hear a whistling sound, and its the small space the cat didn’t cover, when she spread out over the vent!) Does the other cat chase that one? Sometimes, it seems like a certain spot is “base”, like when you’re a kid. “I’m on base! You can’t touch me, as long as I’m on base.” And for some reason, another cat will go along with it. Then when the one is seen, “off base”, they’re fair game!