cat wants privacy in the bathroom

We have a cat who closes herself in the bathroom when she wants to be
alone! For background, we have 2 shelves in the bathroom that the
cats like to lie on - we just keep a towel on them for comfort but
don't try to store anything on them. This is not the bathroom that
has the litter box - it's the "people" bathroom.

Every once in awhile, I'll go upstairs and find the bathroom door
closed. Over time, it's become clear that Smarty purposely shuts the
door behind her, then goes up to chill out on her shelf. We actually
had to get a doorstop so that we could make sure that she didn't lock
herself in the bathroom while we're away or overnight! Does anyone
else's cat do anything like this?