Anything habitually done alone and considered pointless, embarrassing, unwise, or revolting to discuss publicly is a candidate. Most things that go on in the bathroom count. Clipping one's toenails (if one does not wear open-toed shoes); little doodles, drawn with no one watching, to be thrown away after completion; doing laundry in a private washing machine; selecting a toaster setting when preparing breakfast alone.

It's possible that I'm underestimating the sorts of things people subconsciously expect others to pick up on, but I also doubt anyone is signaling when: they (publicly) eat (their own) package of M&Ms with a particular color order in mind; they change lightbulbs; they skip one short story in an anthology; they pick at lint on an article of clothing. Choosing to do these things or not isn't obtrusive, and when they are done at all, there isn't much meaningful variation in how they're done. (Which short story you skip might matter, but out of an entire anthology, it's unlikely that one worth skipping would come up in conversation individually, and I don't think the story-skipping party would be likely to bring it up first.)