It is an extremely crude drawing of a key on the floor. Really, the drawing is so bad, it's ridiculous to think that some prankster thought it would fool you, which it did. Whoever drew this key clearly was employing the most primitive drawing tools available.
You are obviously being fucked with in this stupid jail cell. You expect that you will continue to be fucked with, and it makes you wish you could stab something.
Hey check it out. Every week we'll be revealing some new troll characters from Hiveswap until Act 2 is out. Follow the Troll Call here, and meet the first two here. Expect a few more surprises like this to drop in coming weeks.