Guest took the keys by accident

Good day 

I had a guest who forgot to leave the key when they were leaving. I contacted them but they said they left it on the table which is not true! and they said they will check their bags and contacting me back but they did not.

What can I do? the door of the building and the apartment are secure doors and is expensive to go through the whole new keys. any advice would help me a lot.

Thank you all 


Simply ring them again and ask them to post it back to you when they find it.


Keeping a spare is also advised.