I had a dream last night that I was a rat-man in a dungeon that human adventurers invaded. Lacking the strength to confront them and their hunting-cats directly, I worked to douse their light sources, lead them into ambushes, and change the environment behind them to make it more confusing.
It occurs to me that this is basically the life of a sensor-blip-style random encounter, where they linger in the dungeon near parties rather than moving to confront them directly. And the idea of having random encounters behind the party "clean up" the dungeon is one I hadn't considered before; I know the idea of "dungeon clean-up crew" monsters appeared in some of the older books, but I took it as an ecological consideration, something that happened between adventures rather than during. It amuses me, the image of a crew of ratman janitors armed with a mops and buckets, doing their part to fight the humans by hauling away the bodies, cleaning up the blood, erasing the chalk or charcoal graffiti, trying to put out the oil fires, maybe even moving the furniture around so that they'll have a harder time finding their way back out of the dungeon.
