As SMP says, if it just blew on your plot and you had no idea where it had come from, it's fair game. I've had bits of net, plastic, bucket etc arrive and usually I put them at the end of my plot on the path for a week or so and if they're not claimed they're mine. But since you know whose they are, I think you should return them.
On our site, evicted and given up plots often have stuff left on them but committee members usually get there first as they know in advance. There was a half plot near me taken by a couple who arrived and put hundreds of pounds worth of equipment on the plot - a shed, fruit cage, those slot together compost bins and some other bits and pieces all flat packed and brand new. They never re-appeared so they were eventually sent warning and eviction letters with no response. They weren't on the contact phone number they'd given either so the plot was cleared by committee members. I was there on the day they were doing it and offered a donation to site funds for the shed which was accepted. Don't know what happened to the rest of the stuff though!
Trinity, the allotment officer is not right. If stuff belonging to someone else has fallen on your plot, it is your business. I would write to the allotments officer and tell him that you are going to get enough people together to move the pallets and lay them flat on your neighbour's plot where they can't fall and hurt someone or cause any more damage. Then just do it.