Taking SCARY out of used bikes, one bike at a time.

I’m in Colorado and this is not a recommended place to put your stash… But on a serious note. You should check your brakes before every ride. It’s the “B” in the “A,B,C” check list. In this case you should make sure there isn’t any obstructions in your brake system. This looked like some kind of green paper towel.
Shimano Ultegra Disk Brake Pads
Shimano Ultegra Disk Brake Pads