Bald spot on arm

I noticed a few days ago that Marishka has a bald spot on her arm. It is not round...more long, about an inch long. There is no redness, spots, bumps, scratches or anything. And she pays no attention to it, and I've never caught her scratching or chewing at it.
I can't think of anything she could have scratched it on, and she is an indoor only cat. She doesn't even flinch when I touch it, and fur is obviously growing back. What could be going on?

Could she have pulled her fur out?

Thanks so much for answering, Karen : )
I'm sure she could have...I just figured I would at least have seen her chewing on it, scratching, licking, or something. What would cause a cat to pull fur out, boredom? When I checked last night, it's growing back quickly. I just wish I could figure out what exactly she could have done to it. Is there anything (disease, condition, illness/virus) that would cause hair to fall out on a cat, that I should watch for? Her eyes look clear, her fur seems shiny, and nothing looks sickly that I can tell.

I honestly don't know about illnesses that would cause hair to pull out. You could post and ask on the Health forum. Abby pulls her fur out sometimes due to stress, or food allergies.