Have you ever felt like you’re being watched? Have you ever been up late at night, suddenly gripped by the fear that someone has broken into your home? Well, one Redditor has certainly shared that fear, but for a completely unexpected – and potentially deadly – reason.

On May 2, 2015, a user made a post to the subreddit r/legaladvice about strange post-it notes that they had found around their apartment, starting on April 15.

“I found a yellow post-it note in a handwriting that wasn't mine on my desk reminding me of some errands I had to do, but told literally nobody about,” the post began. “While odd, I chalked it up to something I did in my sleep, thinking maybe in my half-awake state I scrawled it so it didn't appear to be my handwriting. I threw it out and thought little of it.”

However, this was but the start of a series of odd notes that they found over the course of around two weeks. On April 19, yet another note appeared on the back of the Redditor’s desk chair, in the same handwriting. “I was freaked out, but there were no other signs of a break-in, so I set up a web-cam in my house aimed at my desk and used a security-cam app for it to record after detecting movement,” wrote the user.

Another note was found on April 28, with the chilling message: "Our landlord isn't letting me talk to you, but it's important we do."

There was no webcam footage from the night before, and the computer’s recycling bin had been emptied. Something fishy was happening here. On May 1, blank post-it notes appeared on the outside of the user’s door and other doors throughout their apartment complex.

“Do I have any legal recourse here? I have no proof except for the post-its, but those are written by my pen and on my post-it notes, so conceivably I could have faked them,” the user asked the subreddit. “Should I consult my landlord? Those also living in the complex?”

The Redditor dug up an old letter from their landlord and found that the handwriting was “identical”. This led to theories that the landlord was breaking into the apartment, with some suggesting that the user should buy a lock that can only be opened from the inside. However, the April 28 note made this theory pretty doubtful.

Among the mostly confused replies, one user made a genius suggestion. “You mentioned that you have a very unusual narrow bedroom with no windows; is there a chance that you are not getting enough ventilation when you sleep, or that there is a carbon monoxide leak in the building? A cheap CO detector (which you should have anyway) is a fast way to find out. You'll also have really bad headaches.” The original poster replied that they had in fact been having bad headaches, and had a carbon monoxide tester they were yet to take out of its box.

A follow-up post made just over nine hours later confirmed that carbon monoxide was in fact the surprise villain in this creepy story. When the user plugged in a carbon monoxide detector in their bedroom, it gave a reading of 100 ppm. For context, levels of 70 ppm or above can induce symptoms of carbon monoxide poisoning, and levels of 150 ppm or above can be fatal. Neurological symptoms can also arise from inhaling the gas for long periods of time, including memory problems and loss of consciousness. It turned out the Redditor themselves was in fact the one writing the notes, and the carbon monoxide-induced memory loss wiped all recollection of it.

“TL;DR: I had CO poisoning and thought my landlord was stalking me,” concluded the update post. This story is definitely a wake-up call to maybe invest in a carbon monoxide detector – for your own sanity, as well as your health and safety. In this case, someone's life may have been saved by Reddit, as the user has made posts in the years since.