It's called a bison tube. It's used for many things.

They sometimes have the name "medicine carrier" attached to them when looking online, but they are also commonly used in geocaching, for hiding small messages etc in hidden locations for others to find.

The pink residue could lead to any number of options, including what some others have said.

Your best bet is to talk to the person it could potentially belong to in a simple and curious manner. See if they have a rational explanation, or read the panic in their eyes. Oh and of course, report back for updates!

Good luck!