The Whistler

It was Tuesday evening. I had just finished my daily hour at the gym. I put all the equipment away, turned the lights off, and had gathered my belongings. I was just about to open the exit door, when I heard it: a slight whistle. It wasn't super loud, but it was clear enough for me to hear it. I was a little creeped out, as I hadn't seen anyone else in the gym. Then again, the room was quite lonely these days, as its popularity tends to rise and fall. 
Out of further curiosity, I walked toward the men's bathroom door, where I believed the whistle came from. Just to be sure that I was alone, I called out, "Hello?" No answer. I walked closer to the door. "Hello??" I called again. Then, I heard the whistle again, only this time it was a little louder. This gave my brain the confirmation that I wasn't alone. 
For a moment, I had a sinking feeling that there was something wrong, considering that whoever it was in the bathroom only responded with whistling. I looked around the bathroom to see what I could see in the dark, as the light wasn't working for some odd reason. With most of the room covered in darkness, I couldn't find much, but I was at least able to see that there was no one in there. At that point, the feeling I had slipped away. Meh, told myself. Probably just a fucking mouse. 
I got home that night, had dinner, finished my work, relaxed, and slept without incident. No whistling. No weird sounds at all. By the next day, I had nearly forgotten about the whole damn thing entirely, and was doing a fine job of moving on from it. That is, until I turned the TV on at high noon. 
In the middle of a commercial, a breaking news bulletin came on. It was a missing persons case. The person's name and gender had not been identified, but what they did know is that they were last seen at the gym the night before. Holy shit, I thought. I begun to feel nervous, but also relieved that I didn't get caught up in whatever happened in that bathroom. What I saw next on the screen, however, would haunt me for years. 
There was a picture of inside the bathroom. On the wall, written in blood, were those four terrifying words: 
"THE WHISTLER WAS HERE!"
Category:Beings
Category:Disappearances