Hi Surrey,
Like most things in life, there is the right way, and the wrong way to do things!
When a client hands over their keys to you, they assume that you have all the proper procedures and insurance in place. Yes, you should have a written agreement in place with staff, that is legally enforcable, just writing down something and getting them to sign it, will, as Gary's experience shows, not suffice!
You should also have insrance in place to cover the loss or theft of your clients keys, this cover can be obtained from your insurance broker, you should be aware, that there will still be an "excess" to cover any loss.
You should inform the client of any loss or theft straight away, the clients insurance company may well insist on replacing all locks.
Regarding alarm codes, you should state in your contracts, that when a member of your staff leaves, for whatever reason, entry exit codes need to be changed, this will protect you against any future claim, should a non invasive theft take place.
Regards,
Rob