Yes there is residue and the longer it is used the more residue absorbs into the hose material. Be careful what you use an old propane hose for. Definitely not for anything you would eat or drink.

It may be residue from something oily. Use a pre-spotting product. Or, it could just be from leftover bleach residue if you did your whites before doing all your dark clothes ...solution ... do a double rinse on your whites .

In Spanish, the word "concho" can mean raw, vulgar, crude, peasant, person who lives in a rural area, taxi, leftover, remainder, residue, or sediment.

The most prevalent products of a candle burning are carbon dioxide, water vapor, and leftover wax residue.

Put the lock in a freezer for an hour. The gum will become very brittle and chip off easily. If there is any leftover residue, use acetone on a rag to rub it off after the lock returns to room temperature.