I accept that if I leave my seat (like in a public place such as a bus or park)....it's up for grabs as soon as I vacate it. So I have an inner debate of is it worth leaving the seat to go do a task such as a potty break, or should I just endure the discomfort so that I don't lose my seat. Usually the bathroom run wins (which it is run because I didn't want to get up and lose my seat in the first place.) And nobody leaves their stuff on a seat to save it in a public place or it will get stolen.

As far as an office work space.....I'd just say "sorry I've been working here with my stuff and had to run a quick errand...thanks for watching my stuff, now may I have my seat back..." .