Sometimes you just get unlucky. A few days ago I picked up nearly 50 drywall anchor screws (plus exploded plastic packaging) off the road at the end of my driveway. I'm 99% sure they weren't there when I got home (no flats yet) but they were everywhere when I came back from walking the dog. They must have fallen from one of the neighbor's vehicles as it passed by. Luckily I saw them and got them picked up before my wife pulled in a few minutes later.
I've had one flat in a personal vehicle in my driving life (many flats in work vehicles when conducting wildlife biology fieldwork in remote areas) but could have easily had multiple flats in both personal vehicles in one evening.