Let's say that the OP does ask around and still cannot identify who sent the magnet.
Would it be rude of the OP to write a short polite note to the senders saying that she is sorry her memory is so unreliable, but she is just not recognizing their names and asking them to help her remember them?
(I'm thinking that I'd be tempted to just ignore the magnet, but if I was feeling a little feisty and had no luck identifying them by asking around, I might want to go ahead and ask them who they are.)