I bought a used Boxster about 6 months ago. Had a full service, new Michelin Pilots all round, all pads and discs replaced and a new battery all done by the previous owner just before I bought it from a Porsche specialist, and have the receipts. (He was an old man and had to give up driving which is why he sold the car suddenly ;-))
Naturally with a new to me used car I checked the tyre pressures each week or so to check they were holding air. Which they were, so didn't check them again so often.
Car went back to the dealer three times for niggles to be sorted.
Went on a very long journey a couple of weeks ago, so checked the tyre pressures before.
One of the wheels now had an obviously old valve. And the tyre over 50% worn. All the others are nearly as new.
Looks like one of the wheels had been swapped while it was at the dealer. It doesn't have a spare, so not that.
I'm rather miffed - but don't see what I can do as it would have happened months ago.