Most people are not shooting straight up vertically. In practice, they are typically shooting at a high angle such that it creates a long arc and this would not be first time something (or someone) got shot at a distance that no one intended to shoot.

In most cases, "celebratory gunfire" is harmless. In some cases though, people end up injured or dead - there's a decent sized list on Wikipedia [1]. The key issue is, you only have "terminal velocity" at play when shooting up straight at 90 degrees in the air. The more "sideways" you get from that, the more energy will be kept during the fall.

[1]: https://en.wikipedia.org/wiki/Celebratory_gunfire

I had a projectile take out one of the PV panels on my roof. Judging by the hole it was a bullet, though I didn't find anything - if it was a meteorite I would be more excited (and more disappointed not to have found it).

That was through the wall not the ceiling. Bullets fired into air and returning to earth (assuming straight up so little to no horizontal velocity) don’t have the kinetic energy to go through a roof and kill someone.