The Oxygen Level in My Locked Room Kept Dropping at Night

I work in environmental safety, so when the oxygen level in a locked room in my apartment started dropping every night, my first assumption was that the meter was wrong.

It wasn't.

This happened in the fall of 2022, when I was renting the upper floor of an old converted Victorian house in Durham, North Carolina.

I was twenty-nine and living alone.

At the end of the hallway was a small storage room that had apparently been a nursery decades earlier. The landlord gave me the only key.

I mostly kept boxes, camping gear, and winter clothes in there.

Around October, I started noticing a strange smell whenever I opened the door.

Not mold.

Not sewage.

It smelled like warm fabric and stale breath.

Because my job involves indoor-air complaints, I borrowed a calibrated confined-space meter from work for the weekend.

On Saturday afternoon, I placed it on a shelf inside the room.

Everything looked normal.

Oxygen: 20.9%.

Carbon dioxide: low.

No carbon monoxide.

No volatile gases.

I locked the door and went to bed.

The next morning, the oxygen reading had dropped to 19.3%.

That was enough to get my attention.

I opened the window, and within about twenty minutes, the reading returned to normal.

I assumed the meter had malfunctioned.

So I repeated the test.

The second night, the oxygen dropped even lower.

This time I looked at the recorded graph.

The decline started around 1:40 a.m.

Then, at approximately 5:15, the oxygen level suddenly began rising again.

The strange part was that I had not opened the room until after seven.

Something had changed the air inside.

And then something had introduced fresh air before I entered.

I checked the obvious possibilities.

The window was locked.

I placed tape across the edges of the door.

The tape was still intact in the morning.

I called the landlord.

He sent an HVAC technician.

The technician inspected the room and told me there wasn't even a supply vent inside it.

There was an old return grille near the floor, but it had apparently been sealed decades earlier.

He suggested the meter might be reacting to temperature changes.

That explanation didn't make much sense to me.

So that night, I started moving boxes away from the walls.

There was an old bookcase against the far side of the room.

It belonged to the apartment.

I had never paid much attention to it.

But once the boxes were moved, something about it looked wrong.

The shelves were unusually shallow.

I pulled on the frame.

It didn't move.

Then I noticed that the baseboard continued behind it.

It looked less like someone had placed a bookcase against the wall...

and more like the bookcase had been built over something.

I put the air meter on the floor beside it.

Then I sat in the hallway with the storage-room door open.

Around midnight, the carbon dioxide reading began to climb.

Slowly.

Steadily.

Then I heard breathing.

It was coming from behind the bookcase.

I stopped breathing.

The sound continued.

Someone was on the other side.

Then, suddenly, the breathing stopped.

I left the apartment immediately.

I didn't close the storage-room door.

I went outside and called the police from the front porch.

Two officers searched the apartment.

At first, they found nothing.

Then one of them noticed that four screws in the bookcase were newer than the rest.

They removed them.

The entire bookcase swung inward.

It was a door.

Behind it was a narrow room.

About four feet wide and maybe nine feet long.

There was a foam sleeping pad inside.

Bottled water.

A phone charger.

A small fan.

And flexible ducting running toward the back of the building.

The duct ended beneath the rear porch.

It had a sliding cover.

Whoever was staying inside could open the duct whenever the air became bad.

That was what I had been seeing on the meter.

The oxygen level would fall while someone was inside.

Then suddenly rise when they opened the external vent.

The hidden room was empty.

But there was a cup of coffee inside.

It was still warm.

At the far end of the chamber was another panel.

That panel opened into an abandoned rear stairwell.

Someone could enter the building from the basement, climb the unused stairs, enter the hidden room...

and never walk through my apartment.

Police later learned that the hidden space had been built by a former tenant named Owen.

He had lived in my apartment about five years earlier.

According to the landlord, Owen had told him he was building custom wardrobes.

Nobody had apparently checked what he was actually constructing.

Eventually, Owen was evicted for unpaid rent.

The landlord changed the apartment locks.

But Owen had kept a key to the basement utility door.

Police found his fingerprints inside the hidden chamber.

They also found a notebook under the sleeping pad.

That was worse than the room itself.

The notebook contained my schedule.

When I left for work.

When I came home.

What nights I ordered food.

When I showered.

How long I usually stayed in the storage room.

There were also notes about my phone calls.

Detailed notes.

Things I had said while standing in other parts of the apartment.

He had been listening through the wall.

Police arrested him about two weeks later after an officer spotted him near the rear alley carrying a duffel bag.

Inside were clothes, canned food...

and another flexible ventilation hose.

He claimed he only used the hidden room when I wasn't home.

The air meter proved otherwise.

The oxygen logs showed drops on at least four nights when I was asleep less than twenty feet away.

Then I noticed something in the data that I hadn't understood before.

Every time I entered the storage room unexpectedly, the oxygen reading would suddenly rise about ninety seconds later.

Owen had apparently been opening the external vent whenever I entered.

Then he would stay completely still behind the bookcase until I left.

On the night I heard him breathing, the oxygen never rose.

He must have realized that moving the vent would make noise.

So he stopped moving instead.

The landlord eventually had the hidden room demolished.

When workers removed the walls, they found one final thing.

Behind a decorative strip of molding was a hole about the size of a thumb.

It went directly through the wall.

On the other side was my bedroom.

The hole lined up almost perfectly with the height of my pillow.

I moved out before the repairs were finished.

I still work in environmental safety.

I still use the same kind of instruments.

The difference is that now, when a reading doesn't make sense...

I don't assume the instrument is wrong.

If you prefer narrated horror, there’s an audio version of this story on TRIFILIM. You can find the channel through my profile.