Change the locks at your own expense. Keep all keys. If anyone ever wants access to the property then they will have to go through you. If you have been given the okay by the Letting agent or Landlord in writing then don't even tell them, just take that as a green light and get on with it.
Alternatively do what I did. Set up a motion detection camera that emails you when motion is detected. At least then you'll know who it is...
Alternatively do what I did. Set up a motion detection camera that emails you when motion is detected. At least then you'll know who it is...