Tenant sign on door says call the cops

[Dean Klein]

I just bought a duplex 3 weeks ago and have been at the property rehabbing the vacant lower unit. I was outside installing gutter screens and noticed a torn window screen and a slightly open window in the common area. I went up the stairs that my upstairs tenant uses to get to her door because the open window was at the top but not in her apt. I noticed a sign taped to her door that said "If you've made it this far without my permission,  I will call the cops". I need to know my rights to post notices ect, I closed and locked the window and want to make sure I didn't do anything wrong. Her lease clearly states she rents only the space between her apartment walls, not common areas. I am in Wisconsin. I feel like I wasn't in the wrong because I did not go into her apartment, just outside her door.