Man vs Stain

[GroovyJon]

Looks like rust, did you by any chance look at the ceiling? leak at some point possibly? Could have had metal on the carpet that got wet. Could also be a wood stain... Those are my shots in the dark. Nice work on the carpet clean! My gut tells me there was a leak or flood at some point - To be honest If you tried all those things it does not matter, Its most likely permanent. No big deal, you cant win them all but you gave it a shot