We put the keys to homes for sale in lockboxes and attach the lockbox to the house. The lockbox in the picture is electronic and can only be opened by a member of the board of Realtors.
We can only open the boxes with an electronic key that for most of us is an app that runs on our smart phones.
These lock boxes are really secure and the listing agent can even get information from the box about who opened it and when it was opened. Of course we have no way of making sure that the keys went back into the lock box but maybe someday they will make a lock box that can tell us when the keys are missing. The box can also be programmed so that it can only be opened during certain hours.
Not all agents like to use these boxes but it should be up to the seller not the agent.