chunk of paint

It's been a few weeks since I've been on my bike. Last time I rode it, everything was fine. I let my brother ride mine because his had some mechanical issues. I noticed when cleaning my bike last night that a big chunk of paint is missing from the top tube. It is showing the bare aluminum. There doesn't appear to be any dent or anything. I'll take it to the shop to have them check it out.

This is probably impossible to answer, but whats the chances the frame was severely damaged?

I had to store the bike in the garage (usually in the house), so it's possible something fell on it and no one told me. My brother hasn't yet confessed to crashing it, and it seems unlikely if he did that a chunk of paint would be the only damage.

The chunk is about an inch by half an inch big, on the right side of the top tube.

He probably leaned the bike against something and scraped it, not crashed it. I lock my bike in a lot of places, and every time I lean it against something I probably scrape a bit of paint off it. You should see how much exposed steel and scratches are on my top tube, but it's intact and fine. I doubt there is anything mechanically wrong.

If there is no dent, probably no structural damage. You can check the joints for cracks as well, but probably just a scrape of the paint.