Doctor Help, what's this?

Hi,

My husband has had these dark red marks showing up on his back for years. I thought maybe it's from his shirt, or office chair or something else he's leaning on, but that's not it. It's so symmetric.

It's not a rash and he doesn't know/feel anything on his back. Any doctors out there have an idea of what it is?

If he's not been leaning on anything or sleeping they usually fade away.

The indents are from the bed sheets but the red marks are weird. They are not raised or sunken in/bumpy. Just red.