If you're really intent on finding what caused it, measure the hole position on the outside, then crawl up in there and locate it on the inside. Cut a small opening. I bet you find it resting on the insulation (if there is any in that area). Otherwise, it probably fell into the basement. In which case, it wouldn't be worth it to open it up.
But I agree, if it were a bullet, you'd know. Likely, it was a rounded rock that was sitting nice and comfy in a truck tire tread and decided to let loose. But then again, you'd have heard the impact being on the driver side.
Or like someone mentioned, a wrist rocket projectile, like a ball bearing, marble (do they even make marbles anymore?) or something similarly shaped. If that's the case, the little darling responsible was probably aiming for your mirror and missed. Do you recall ticking off any kids lately?