My last apartment was managed by an extremely shady and unprofessional office of a real estate franchise.
Prior to our signing the lease, we asked both the showing agent and the signing agent/manager a particular question, and they both lied blatantly. Unfortunately, the 'four corners' of the lease were silent on this seemingly straightford issue so when I called them three days into the lease to arrange resolution and they lied and said they'd never said it, we had little recourse but a lengthy, uncertain civil complaints process, which was just not worth it given that we had no proof (lesson learned). They were bizarrely unprofessional about the whole thing, contradicting themselves with new lies, sneering, mocking, throwing personal insults and whatnot.
Anyway, about a week after moving in, I came home unexpectedly early on a weekday and saw a man dressed like a real estate agent in the building's common stairway near our apartment door. I didn't recognise hime and mightn't have thought anything of it except that he sort of started when he saw me, and had a 'deer in headlights' sort of look before composing himself, mumble-babbling something then hurriedly leaving the building. When I went inside, I could smell god-awful aftershave, and some private documents I had left on the dining table that morning were fanned out. Two $50 notes from a birthday card were missing from the kitchen bench. No aircon, no windows or doors nearby, no fan, and I had left well after my partner that morning.
I was unnerved, but against my intuition I talked myself into believing my partner's more reasonable theory that I was tired from the move and unpacking and misremembering how/where I had left everything.
The following week (it was a weekday) my partner had just left for work in the car. I had an unexpected day off and was sitting on the living room couch in just my underwear drinking earl grey and moisturising whilst I caught up on 'The Good Wife', when I heard a key turn in the lock.
My cat growled and ran into the bedroom, but I didn't move, thinking it was my partner returning, having forgotten something.
Two seconds later, that man opened the door and casually strolled into my apartment, staring at his phone! With his shoulder still propping my door open, he set his smoothie down on the corner of my occasional, and was reaching into a bag hanging from his phone arm. My heart almost jumped out of my throat. I screeched and grabbed for my couch throw to cover up. He almost jumped out of his skin - looked up, shocked, grabbed his smoothie and literally just backed out, really quickly shutting the door in front of him without saying anything.
I heard him go upstairs, grabbed my clothes and waited. After about two minutes I could hear him trying to slowly, quietly come back down the stairs (they were creaky as).
I opened the door wide, my heart thumping in my chest the whole time, and said, really loudly, "Who are you?! How do you have a key to our apartment?!"
Him, rushing past: "Oh, sorry, sorry, I'm about to do an open house upstairs and I opened your door by accident!"
"How?"
"Huh?"
"How did you open our door?"
"I used the master key."
"There is no master key for the apartments, there us only a master key for the building; the apartment locks are all independent." (The building had about 40 apartments - some were owner occupied, some leased privately, and the rest were leased by different real estate entities).
"Oh, well, I must have picked up your key by mistake."
"So you accidentally took my apartment key, and then also happened to accidentally come to this side of the building, this landing, this exact door - out of all the apartments in this building that you could have 'accidentally' tried to unlock?"
He just sort of sneered and walked out.
I didn't hear him unlock or enter any apartments whilst he was upstairs and no open house was held that day. I actually went upstairs and asked the other residents if any of them had had an open house scheduled and none of them had.
I don't know what their intentions were. I suspect that because they knew they were being super unethical, they may have been concerned that we would take our dispute to the Civil & Administrative Tribunal or TAAS or Tenant's Union and wanted to find or manufacture evidence that we were breaking the terms of our lease (or the law) as leverage. Ordinarily I'd say that sounds paranoid but these people were so dishonest and unprofessional that I would not put it past them.
Which is all a long, roundabout way of saying, change your locks when you move in to a leased property to minimise the number of people who have access to your home, and get a security system.