Hi @Frances11,
I had a case two years ago, where guests lost a key (their whole rucksack with the key inside got stolen at the Oktoberfest in Munich) and after questionning the guests if there was anything inside the rucksack, where you could track back my address, i.e. a print out from the airbnb travel plan or any other documents. As they denied, just clothes inside, because they wanted to buy some of these festival outfits and needed to put their other clothes somewhere, after this I decided to not change the lock.
At least they paid for the key, which was about 10 Euros.
In your case, the keys are still there and if the chinese couple is already back in China, I would either ask them to send the keys with ordinary postal mail - and yes for security purposes I would choose a friend's address to recieve the parcel. Or you also could tell them to throw the keys away into the rubbish bin and I would order another set of keys.
In the first case the guests would pay for the shipping fee and in the last case you can collect the money for re-doing the keys through the airbnb mediation center, they open a case and the guest has to agree for the charge.
If there is nothing attached to the keys like little tags with address or name, I wouldn't be too worried and would not change the lock (they probably don't go to next theft to tell him "these are the keys from Frances and the address is xxxx"), but of course this is a personal opinion.
But if you don't live in your own property, there might be some house rules attached to your lease contract, which oblige you to change the lock in specific cases, if this case I would contact your property manager and ask him about the lock, and if you must change it, then I would try to get these extra cost back from the guests (also through the mediations center, may be with a kind note "unfortunately I had to change the lock because of the property security rules ..." or similar.
Good luck
