May I suggest you return to place found and leave high up on wall/fence - visible for parent/carer who probably will retrace their steps when realise it's missing!
A friend of mine lost her sons first shoe, fell from pram. Devastated for sentimental reasons. Retraced steps for days but eventually found it high up on a wall post. Must have been hiding in leaves or somewhere on ground for days. Until a kind person put it in full view.
Hope that helps and trainer is re-united with owner!