Maybe she came over to check on her couch?

She must have been upset you didn't stop ******* to greet her. Quite impolite.

Did you forget to put a sock on the door?

Your husband needs to have a word with his mother about stopping by without calling. And maybe he needs to take the key back if she's going to invade your privacy that way. Unless you have kids or you had the blinds open, you have every right to get busy wherever you want in the privacy of your own home. And if MIL can't respect that, it's her problem. (I once read a story about a woman whose family would often come over without asking, even though she repeatedly asked them not to. So the next time she did it, she and her husband pretended they were getting busy. It stopped her family from making unannounced visits. Too bad your MIL is not so considerate.)

Change your locks now... then have a long talk with your MIL about trespassing.

Change the locks

I wish you could File that under unlawful entry

invite her over next time

Picture this: we were both butt-naked banging on the bathroom floor.

You should check out JustNOMIL on reddit. They're your people probably