2001 honda civic bolt (X post from /r/askamechanic)

I was driving yesterday and a bolt fell out of the car. the car had been making a faint rattling noise for about 3 weeks. Drives fine, no warning lights. Still rattles a little but the rattle sounds a bit different now. I tried to see where it fell out of and also tried matching the size with on the car but i couldn't locate where it came out of. picture of the bolt.

I also replaced the front brake rotor and pad yesterday but i know its not from the brake job i did. Couple months ago, i removed the serpentine belt and harmonic balancer out ( also loosened the alternator to remove the belt). The alternator bolt loosened over time and it fried the ECU. Bought a used ECU from ebay and took it to honda dealer to get it reflashed and installed.