One part of the center where I work is like a bonus room. It is a converted garage, but still has the big double garage doors as one wall. I have worked there since 1994 and never had anything like this happen before or since - just on one day. And then I found out the next day that it happened to my coworker too, hours earlier. This must be about 6 years ago.
To set the scene - it's winter. An extremely cold, bitter winter day. There is fresh snow on the ground from the morning's dusting. I am sitting about 2 feet away from the garage door, in a chair I sit in every day, so that I can observe all the children playing.
Suddenly, behind me, there is a very loud series of knocks on the garage door. Very loud, very sharp, very striking. I jumped out of my chair and turned around, cos the sudden jarring sound scared the crap right outta me. The kids stopped playing and all turned to look in the same direction, where I was looking. They were saying things like 'WHAT WAS THAT?'. I immediately jumped up on my chair to look out the garage windows, expecting to see someone standing there, like the neighbour teens that used to go to the center, maybe banging on the door to scare us. But no one was there. I peered down at the snow and I could not see any footprints either. NONE. ANYWHERE. None anywhere on the driveway at all.
So, that was weird.
Some of the kids were quite scared by it and were jittery. So I decided to pop my coat on and walk outside to see if maybe someone had thrown an ice ball at the garage door, or something. A rock. Anything. I was going to give the neighbour kids heck for scaring my group of kids if they were out there.
So I come around the corner of the garage and step very carefully - but there is NOTHING in the snow at all. Not a dent or a bump, or anything to indicate that maybe a snowball or iceball hit the door and then blasted into smithereens before hitting the snow. There were no snow marks on the door itself either, like you might expect to see from a thrown snow ball. It was so odd.
The whole driveway was clear, undisturbed. I felt like I was suddenly on CSI, trying to solve a case, but I could not figure it out at all. I just hoped it wouldnt happen again or kids were going to start crying. It was THAT loud.
But it didnt happen again and we went down to the main center an hour or so later. No big deal. Just an unsolved mystery. I never said anything about it, and neither did the kids.
So, the next day, I brought it up with a coworker. I said "I dont know if the neighbour kids did it or what, but it was freaking loud, and I couldnt see any foot prints or anything at all". My coworker was staring at me like I was from Mars, so I felt stupid. She said 'You need to talk to J" (another coworker). So she calls her over, and says to the other woman "Tell her what happened to you in the garage yesterday morning".
THE SAME DAMN THING!
She was sitting there, la de da, watching the kids, when there was suddenly 3 or 4 really loud knocks on the door behind her. It shocked her because it was so sudden and loud, and she hopped up on a chair to look out the window, and see who was goofing off. But she saw nothing, no one, and there were no foot prints around. She did not go outside, but she looked around a lot, she said, because she figured someone HAD to be out there, but couldnt see how they could get near the door without leaving footprints in the deep snow. It was also so cold, the snow was crunchy.
Anyway, so then I told her what happened to me in the afternoon, and she couldnt believe it. So here we were, exchanging stories in front of our one witness, who had nothing to do with any of it, and was the first and only person both of us told. It was strange.
When I talked about it on a local paranormal group, they said it was probably just the door cracking because it was so cold.
Okay - plausible. But how is it that in almost 20 years of me working there, and over 10 years of the other girl working there, and both of us taking groups of children up there year after year (we swap days to be fair), that neither of us has EVER had that happen before, and then both of us on the same day, and then never again since? Every winter we have plenty of days that drop below -30C, and we've had days below -40 and even below -50 with windchill a fair number of times. So why that one day, and never again? It also was not a crack. I described it as 4 or 5 sturdy direct knocks, like a person would with their knuckles on a door, but very deliberate, spaced apart, and LOUD. She said 3 or 4 knocks, and immediately assumed 'person', not 'oh man, was that a crack or a snap?'. A serious of knocks, nothing more, nothing less.