It never went on my P11d, and was part of the ‘uniform’ of a dock worker, hi vis safety gear/boots etc. I’ve just accumulated lots of it over the years!
I suppose they could just say that used workwear was of zero value so wouldn’t be on it anyway.
But in general, as above I’d suspect it’s more down to it still being company property. I doubt they would really GAS, but you never know they might have an ebay saved search setup looking for people selling off their kit for a bit of income and just getting more from stores.
Just a thought, what’s stopping someone from buying it and then pretending to be an employee of the company for nefarious reasons?
If they let any old person in a high vis jacket on site then they’ve got security issues!