Absolutely, happy to help.

It’s really hard to tell from the picture, but the only time I’ve not seen sheathing used was on a shed and screened in porch, stuff that doesn’t really matter too much. I would be really surprised if there wasn’t any. I’m not am engineer, so I can’t say for sure, but I was always told the sheathing does provide a bit of rigidity and shear strength to the structure.

Sheathing is typically OSB so it is possible that it was a weak spot that crumbled out or the builder just cut it a bit short, that wouldn’t be a super weird thing to see and probably the most likely. It also depends on how old the house is. I actually don’t know when they first started using OSB or Plywood as sheathing but it could be completely different if its pretty old, I’ve seen a couple houses built circa 1910 that just had boards. If you’re really gentle with the siding, you might be able to pull it back a bit to get a better look.