Disc brake pad spacer
After looking at this picture, internet gurus began offering explanations of what kind of key this was. Perhaps it’s used to open a thermostat cover, or maybe this style of key is for specific doors that keep the world’s mysteries under wraps. However, those people were unequivocally wrong.
This is a disc brake pad spacer. Basically, it separates a roto from a braking mechanism to stop a device from tearing itself to shreds when the brake is engaged. As to what rotor and brake this thing created space between, nobody, not even the OP, could provide an adequate answer. We need more details!