Mystery little bolt fell off near rear axle Hi All! Please help with advice. I was doing some regular brush cleaning on my new Diamond Mission Pro bike that I purchased off the diamondback website (less then month old). A little bolt near rear axle that screws into the frame just fell off under my brush when I felt it was loose and touched it with my hand…

Should I try to replace anything under the warranty?

Thanks a lot in advance!
This bolt is used to hold the nut on Shimano E-Thru rear axles, it's usually a part of rear derailleur hanger or drive side dropout. I think it prevents the nut from rotating and the fact that it snapped off might be a symptom of loose rear wheel axle. Sections of recess around the bolt without paint might be where it touched the frame.

If you still have the frame/bike under warranty, I advise to contact the place you purchased it from. Even if the issue comes down to user error (loose axle), they will probably help to extract the remaining bolt section and source a replacement.

Alternatively, you can just make a slot for flat screwdriver and use the leftover as a grub screw (that's what my dropout uses, for example) or unthread it and use any suitable bolt instead.