I've cleaned carpets for 35+ years now, I've never seen damage like this before.
I thought it was probably sun damage being in front of the patio door.
But the damage is only by the heat vent.
The other patio door next to this one the carpet is normal.
I thought maybe water got under it and damaged it, new owner said the floor had no damage.
A dog scratching the carpet would fray it and also damage the backing.
It's almost similar to toilet bowl acid damage, but wouldn't make sense in front of the door.
I can only figure it must be sun damage, idk

Maybe the sun is magnified through the window at certain times of day/angle? Or green house effect when a curtain is closed and register is pumping heat behind curtain as well?

Why would sun damage only effect the first three inches of carpet. Being close to the door frame has to have something to do with it and moisture would surely be a safe guess right?

curtains closed the rest of the room doesn't get that intense sun

it all seems to stop at the curtain / blind line.

Maybe somebody was staining or varnishing the wood or even removing varnish and split something on it ? My eyes ain't much but looks like light spots on wood as well.

I know some that urinate in them catch basins,,,,,,,,,

Probably what happened, it sure is crazy

Probably what happened

My machine didn't find any pet hair, I'm guessing the last owners had no pets.
I did a rental one day, I told her my machine found a lot of pet hair, she said she lived there five years and no pet.
Goes to show there wasn't a good vacuum used in those five years
Also no yellow spots or odor, pretty sure there were no pets there

Ya I think sun damage on the varnish wood, no varnish on the carpet, it would still be there

who said anything about pets,,,thinking upstate or 160, ctrap....Others,,,,,,,,,,......
No yellow, they would have been clear by then

Looks like it had something laying on it.

Tracking in ice melt in winter?

Rotten.

Boco carpet design........guessing he peed in a corner or too...........

I have seen carpet beetle damage like that before. Further closer inspection should reveal any active beetle infestation.

Maybe just wear and tear from walking in and out the door. Standing with half their foot on the wood frame and the other half on the carpet, could be putting more weight on the carpet and kind of twisting their foot going in and out the door?
Is that a sliding door? If it is, people like to only open them half way to go in and out and walk through the door sideways. Possibly putting more wear on the carpet in that spot.

Or they have something corrosive like salt on their steps and it’s eating away the carpet.

Bobby J is smart.

The door that opens is the left one, this one doesn't open

That door doesn't open, the one next to it does, carpet is normal there