Does the discolouration extend a fair way down the tufts or is it confined to just the tips?
Bit difficult to get a feel for it from the pics unfortunately
The size of area covered and the shape of the spots doesn't seem to be consistent with dog pee or even a drink spill, weird.
I suppose it may have been a cup of tea that was dropped and landed the right way up and just splattered out in all directions, but normally there's a puddle at the epicentre of the splashes.
Do you think they have put something on it but lied to you? Sounds a bit suss, I'd drop it like a hot potato if their prime concern is getting rid of these marks.
Have they given you any idea of how the marks got there or just said they haven't got a clue? It's very rare that they don't have any clue whatsoever, normally they can tell you exactly what it was and how it happened.
What location had the rug been used in?