Legal Question in Landlord & Tenant Law in Michigan

You have no idea why the keys were there or if they actually accessed your apartment. Also, you don't know what their explanation or response would be, and if they would even tell the truth. Any complaints to them should definitely be placed in writing. It would also be your word against theirs that you actually found the keys in your doorknob, versus on the ground, etc.

For this particular situation, you should start at the top - not just the onsite manager or the person who reviews applications. Contact the owner of the apartment complex or the management company and request an in-person meeting to discuss this issue. Treat it very seriously, as it may be a serious violation of your privacy or sense of well-being, depending on what their reasoning is (and if it's even plausible). It could have been an honest mistake - they were going to enter another apartment, put the key in the wrong door, got distracted, and left them. Was the right key in your door - i.e., would it have opened? If yes, that likely means it wasn't a complete mistake. And it is a tad odd that they would "forget" the keys and leave them there long enough for someone else to find, instead of going to look for their lost keys.

You also don't know if it was a maintenance person, or perhaps the manager attempting to show an apartment to someone else (and made a mistake going to yours).

Unless you address the issue, your mind will run wild and you will assume the worst. I don't know that a court would find that this incident alone would justify you breaking your lease. However, a judge's opinion would be heavily based on the landlord's reasoning for the keys being in your apartment door lock, and whether or not someone actually accessed your apartment (even if they lie in court, the judge would be able to see through it).

If I were you, I'd address the issue immediately, unless you just want to move out since your lease is up in a few weeks anyways, and address it when you leave. I think you'd feel better addressing it now and putting it behind you. But, if you feel uncomfortable or feel that addressing it would make it worse, then wait until you have already moved out of the premises. Put everything in writing.