I forgot about this til I saw your thread - I've been on the other end of this - at least my kid a few years ago.
I'm coaching soccer and my kid's on the team. He practiced in a field area with multiple fields. So we're leaving and his ball is gone. I'm looking around and can't find it anywhere when I see his ball over around the seating area and tell him to go get it. He runs back over and says its not ours. There's about 7 other soccer fields and people coming and going, so I figured okay, maybe someone on his team got it or something...
So later that week the soccer league director emails me. A soccer mom had my kid's ball and wanted to trade back with me - she assumed I had her kid's ball, which I didn't - so she asked the director if I'd meet with her the next practice day to trade back. I let him know I didn't have the ball but would like mine back...
She refused to meet and give us the ball back...I had no idea who the woman was, so I didn't know who to look for to confront, the director said she stopped email contact with him and I never got my kid's ball back...
All in all, I decided it wasn't worth getting worked up about, this insane woman and a $15 soccer ball