I think the people who live upstairs from me play the same game as Joel Spolsky's upstairs neighbours. It's amazing how often people drop things, things that rattle and skitter across the floor. You don't notice it if you live in a detached dwelling, but in an apartment you become well-attuned to your neighbours' noises.
I like to try to determine what my upstairs neighbours are doing based on the sounds they make. There is one noise, which sounds to me as if it is coming from their master bedroom (their unit has the same layout as mine), that sounds like "TICK! Tick tick tick tick tick tick tick tick!" The only explanation that makes sense is that they sleep on a giant roulette wheel, and the sound we hear is the ball bouncing around the wheel until it comes to a rest.
Another noise we hear, sometimes, is the sound of water running. I'm not talking about a faucet in a sink or someone flushing a toilet, or even filling a bathtub; it's more like someone draining a bathtub by picking it up and pouring it into another bathtub. The volume of water involved and the length of time precludes anyone but Bob McKenzie from urinating that long. Given that this only happens around 3:00 AM I can only conclude that they are harbouring an escaped killer whale, or perhaps a mermaid, and they are trying to keep it wet without attracting too much attention. There is a security camera in the pool area or else I'm sure they'd just use that.
The last mystery sound is the strange alarm noise that plays constantly. It sounds like "BEEP beep beep beep beep". One long beep followed by four shorter beeps. This is not a typical clock radio alarm, and it doesn't sound at predictable times, as an alarm would. I hear this alarm every day, throughout the day, weekday, weekend, middle of the day or middle of the night, rain or shine. The alarm sounds at random times; sometimes several hours of silence will elapse between episodes, and sometimes it sounds off multiple times in a few minutes. I did some googling and found that the only product that makes such a beep is a perimeter alarm, used to alert people when someone, or something, crosses a boundary that should not be crossed. The boundary in question, in this case, is the perimeter near the whale/mermaid pen, and the thing that shouldn't cross the boundary is a pet narcoleptic sleepwalking hamster. The poor little thing falls asleep and wanders off, and it's well known that mermaids and orcas will eat hamsters if they approach too closely. It's good to see that people still care for their pets in days when it's often cheaper to buy a new pet than to feed your existing one.
Even with the mystery of the noises solved, it's still annoying to listen to them. Studies have shown that noise increases stress; This is why I'm planning to fill their entire apartment with shaving foam, or perhaps sprayable cheese, in order to sound-proof it. I'm sure their other neighbours will thank me.