Random piece of plastic
This appears to be a disc brake spacer (block).

Such a spacer may be inserted in the disc brake caliper between the brake pads. When the wheels are removed, and hence the brake rotor, an inadvertent pull on the brake lever might move the pistons* so that the rotor cannot be inserted any more. It is even conceivable that the pistons might be dislodged entirely – with rather messy consequences. A spacer block inserted in the caliper prevents such mishaps.

Swifty pointed out in the comments the blocks need to be scrupulously clean lest brake pads get contaminated with oils transferred during insertion.

*In a hydraulic disc brake the brake pads are pushed from either side into the rotor by hydraulic pistons. After releasing the levers the pistons do not move back entirely. This is due to a mechanism that automatically compensated for pads and rotor getting thinner from wear.