A few nights ago, mine started beeping in the middle of my sleep and I swear I heard a bunch of really fast beeps right before I woke up and I was like *wtf was that", and 5 seconds later a single beep and I was like "oh".
On US Navy ships, one of the first indications of a casualty is that ventilation stops, either via loss of electrical power or on purpose to prevent the spread of smoke. I now have an acute response when any fans suddenly turn off.
I'm so used to the noise of cars that I can barely sleep without it, due to my bed standing literally five meters away from the main road as a kid.
Now it's also impossible to sleep without the sound of the fan of my PI rumouring because it's probably dusted inside the mechanism, like the Servers fans. And if I don't hear those HDDs spinning up at 4 am for the nightly backup, sleep can wait.
I'm pretty sure this is a thing, I sleep with a fan so my girlfriend does too, I often get up a lot in the middle of the night and she hardly wakes, and we even sleep with the TV on sometimes though i don't like it. One time though I accidentally unplugged the fan and our white noise was gone and she shot straight up super startled like I had set off fireworks
Almost me, except I had to get up and check the window, when it sounded like my cat clawing at it. It wasn't my cat. It was a middle aged woman who was cutting my bedroom screen open. After opening my curtains she ran away, so nothing happened. But I had to get my screen fixed. And I was scared to sleep in my bedroom for a month, and with lights on.