Beat me to it!It's from your garage door. Good luck.
If it is from the car then I would agree. Looking at the piece it doesn't look like its been used because you would typically see some wear from the bolts being tightened around the holes.Although this is an extreme case, I have witnessed 'extra' parts left inside a car during assembly. But those have been nuts & bolts that were dropped and not retrieved. I have a hard time believing that this thing has come unbolted from the car. It more likely got left underneath, while another one was actually installed.