The Mysterious Red Backpack

Travelling from Zurich main station to La Punt went great. We managed to catch all the trains that we needed to, and apart from a few mildly annoyed conducturs, inquisitive passengers, and a forgotten sleeping bag, we arrived all safe and sound in La Punt.
However, at the station in La Punt, I found a red backpack that seemed to belong to no one lying on the ground. Thinking nothing of it, I was convinced that the backpack belonged to a participant, so I took it to the house with me. After we arrived and all the kids were checked in, I tried calling the station in Pontresina to see whether they had found the lost sleeping bag. They didn't have it, but they mentioned that some other people called because they were missing a red backpack. I thought that I had counted two and two together correctly, and told them that the missing backpack was with us. But when I went back to the luggage, the backpack was gone. I looked for it in all the participant's room, without success.
A bit later, the owners of the missing backpack called that they wanted to pick it up. Now I was in trouble: What should I tell the owners? That we had it and lost it? That the backpack that we had found actually proved to belong to a participant? I went for that last option, even though I didn't find it in the rooms, and I told that to the probably-not-owners.
About a an hour ago, the people with the missing red backpack texted me to say that they found their backpack on the train, after it had gone to Scuol and back again. This leaves us with just one question. Where is the red backpack?
Gabriel