Chances are that RM tried to deliver and the recipient wasnt in, they will claim to have left a card but probably wont of, ask them to call at their local sorting office, it should be there.
Or wait and it will be returned to you with a not called for sticker on.
Did you put your return address on it?
We send a lot of parcels through work and this happens far too often, but parcels rarely disappear forever.