Break in??

This morning Nate woke me up just after 6 am and told me he just heard our front door unlock and swing open. Nate is a VERY light sleeper and he said he was laying in bed and jumped out of bed as soon as he heard it. Our deadbolt makes a screechy sound when it opens so that is probably what woke him up. He jumped right out of bed and grabbed the best weapon in our room he could find - which turned out to be our CRUCIAL CONVERSATIONS mug... the one we keep change in. Too bad we don't have any baseball bats hidden under our bed (but we might just have to get one now). Any weapon is better than none I guess. But then he didn't see anyone in our home. Our door was unlocked though when he got up and we both were sure we locked it last night. We only have 3 keys for our house...mine, his, and my parents have a spare one tucked away...so we were so confused as to how someone could get it. We called my parents and they didn't try to get in. Nate and I were talking about all the things that could happen to our home if someone had an extra key to it. He was determined to go buy a gun later that day. We never changed our locks when we moved in so we thought maybe some random key to our house was floating around somewhere. We knew the maintenance crew didn't have keys to each condo because we asked that question before. Nate didn't go to work because we wanted to change our locks immediately. So early this morning we went to a locksmith store and they changed our locks for us. We felt a lot better that no one could get in our condo. So about 5 minutes after we got home there was a knock on our door. We looked through the peep-hole to find a man standing there, holding a set of keys. He told us he is visiting his son who lives diagonal from us and this morning at 6am he was trying to get into his son's place and he unlocked the door like normal...but when he opened it up, he noticed he was in the wrong place! So our mystery was solved! But it was a little disturbing that his key unlocked our door too! Maybe all the locks in our building are the same. I'm glad our locks are changed now so that doesn't matter for us anymore. But if you live in a condo or apartment, you might want to make sure your neighbor's key can't unlock your door too!