Polyprop carpets can change drastically in colour with uv light and quite quick to. Havnt you seen red ones go black , especially the cheap ribbed entrance matting.
Had to visit and report on a ' red wine stain ' for an insurance company, got there it was a holiday cottage , cheap polyprop loop ,red in colour , south facing window. Pulled rug back and the fading was significant but there was an area that looked like a stain between the rug and window as it was redder than the rest. No transfer to cloth , nothing visible on the felt back or underlay , no staining on the wool rug. But there was an artificial pot plant on the window sill casting a shadow that matched the 'stain'. Client adamant the stain appeared suddenly but I told her I would be reporting my opinion which was uv light fading and not accidental damage.
Mike