Landlord agent enter without notice, what can we do?

Question

The landlord is selling the apt we currently rent. We try to coorporate whenever they notify us in advance. Yester they call one hour before showing, we were very unhappy but let them in and told them no next time. Today we came home and found the light and fan was on, and A/C was used. Apparently the agent came today but he totally deny it and the landlord said he has no knowledge about it. What can we do?

Answer 1

Unless you can prove that the agent entered (or someone else), not much.  

Answer 2

Talk to the landlord and let them know you would like the Lock Box taken off.  This way the agent doesn't have a key to the house and cannot get in unless you are there.