A Neighbourly Dilemma

our neighbours' bikes got stolen and they're blaming us for it??!! It's coz in our new townhouse that we're renting, both of us share a common staircase that is enclosed which leads to the carpark. Now, we hardly ever use that door except maybe once a week or once a fortnight when we throw rubbish. But most of the time, we throw our rubbish on sunday nights, when the bins are brought round to the front along the road in front of our house. So really, we hardly use the door at all, unlike our neighbours. Anyway they've always reminded us to lock the door that leads to the carpark and they even stuck up a sign to close and lock the door on that door itself after we moved in, which I'm pretty sure we have. They keep their bikes in the stairwell so that's why they like it locked. Anyway today while i was upstairs I could hear the father screaming and shouting, (the family is pretty loud and you can always hear them screaming and shouting). Anyway I overheard and knew that their bikes got stolen probably coz someone left the carpark door open and the stairwell door unlocked. Then I heard their son say, .....the neighbours are home, their back door is open. Next thing I know, the next door neighbour is at our door, telling us that their bikes got stolen and reminding us to lock the door. Indirectly she was accusing us of leaving the door unlocked so that people could get in to steal their bikes. I tried explaining to her that we hardly use that door but she insisted that they all knew how to lock the door, including her children. So anyway I apologised for it (not sure why I did anyway), assured her that I would remember to lock the door in the future and said how sorry I was that their bike got stolen. But I have been deeply troubled since then, coz knowing how little we use that door, I'm pretty sure it wasn't our fault. I distinctly remember that the last time I went down to the carpark was last Wed or Thurs and I'm pretty sure they would have gone down to the carpark between last Wed and now. Anyway I'm still pretty disturbed by this coz I'm pretty sure it's not us, but at the same time I don't want our neighbours to keep thinking that it's our fault, but at the same time, I don't really want to make matters worse by suggesting that it's their fault. So how?