The random spluge-stain you see under a UV light (generally on a motel bed).

Mystery Taco: When a woman secretly inserts taco meat into her own vagina prior to sex, creating a delicious surprise for her partner, hopefully to be discovered during cunnilingus.

This is the female version of the glory hole. The hidden woman makes her taco mysterious.

Finding an unknown object and t-bagging it.

Mystery Unknown One, it's a term used for people disguised as an unknown one, and are a mystery for people. It's also used to tag people who never talk about their selves. It's also a name for a Tf2 Server Community's Admin; The name of the community is AoDRP, he gets bullied by spamming Tatsumaki's Rep commands.

The unexplainable, but rather enjoyable rush of wind that is emitted up the stairwells leading to the subway system.

An 1850's term to reference 'sausages'. The 'mystery' was due to the fact that nobody but the maker of the sausages would know what was in them, and the 'bags' were the guts which contained the chopped meat.