Another "What's that on my carpet" thread

I’ve got an off-white carpet that gets small orangish spots on it in certain places. Looks like what rusty water drips would leave.

I’m a bit of a clean freak. Nobody wears shoes on the carpet and I keep it vacuumed regularly. No food in the living room. I have a drink on the coffee table sometimes. But the spots appear both on the other side of the coffee table and in parts of the room where I never even walk with drinks.

In the 12+ years I’ve lived in my house I never did figure out what was causing them. When I get around to cleaning them every 6 months or so they clean up fairly well with carpet cleaner.

I don’t see many bugs in my house except spiders. Do they leave spots?