someone suggested the OP replace the lock, Id say this is a good interim measure, but inform the landlord, the landlord may want to replace like for like though, some lo ks are not cheap. If the lock was cheap Id consider upgrading it, but I would not expect the landlord to pay (that suggestion, even asking is ridiculous) as leaving keys out unattended is careless, even hidden if in an envelope, I wouldnt leave car keys or my bank cards out like that, nor house keys.
Similarily I was asked to leave keys in a letterbox at an apartment that was externally on the building, probably easier for An Post but its just an aluminium box, I could have easily fished keys out had I left them there, If likely even without tools to rip the flaps off. Id prefer aluminium letterbox to nothing though as unless there is damage, it suggests no one has accessed it. As for an open set up, Id be a bit concerned if anything of importance was turning up.