A tourist caught their eye on something shining on the sidewalk, just under their line of sight.
Coming upon it, they realized that is an ordinary house key that someone probably dropped.
They picked it up and put in in their pocket, making sure to drop it off at the lost and found after they were finished with their errands.
After they were finished, they came back home, and it was not too long after they became settled that they realized that they had left without turning the key in, but they did not really concern themselves with the situation any more; essentially forgetting about the key in their pocket.
One day, they came back to their house and found that their house key was missing, but going through their jeans pocket, to their pleasant surprise they found the key that they had picked up a way back.
Out of sheer desperation they put the key in the lock, and of course it worked