For me, it was probably this one time when I was in high school. I had gotten to school very early (5:30 AM, school didn't start til 7:30) so that myself and two of my other buddies from the track team could get in some early hurdle technique practice. We had been told that a janitor arrived around that time on that particular day of the week, so we knew we'd be able to get in through the entrance in the back of the school near the gym/laundry entrance.
Well, we get there and find that the door is already unlocked, so we head inside. There are a few lights turned on leading down the hall toward the gym, and so we go down there and try to unlock the door but the doors are locked and we can't get in. This is odd, as the janitor was supposed to have known that we were going to be coming and should have had the door unlocked for us so that we could get in and practice.
So we decided to go walk around and find the janitor. We figured it wouldn't be hard as there weren't many lights on, but as we walked around the lit areas of the school we couldn't find the guy at all. Eventually we started walking around the dark portions of the school as well (this was in the winter time, sun wouldn't be up til past 7). We went up to the 2nd floor past the office and down the hall, and it was pitch black. But suddenly we heard a voice talking, as though it's in a conversation. We couldn't make out any of what it was saying, but then it addressed us:
VOICE: "Oh, I didn't realize you were here."
So this was kind of weird because the voice was literally coming from the darkness. We see a guy start to walk toward us but his face is kind of obscured because it's super dark. The only light coming in is the little light that the moon and street lamps are giving through the glass front doors down the hall.
TIM: "Um, yeah... well, could you come and unlock the gym for us?"
VOICE: "I'll be there in a second."
So, the person turns around and walks back into the darkness, and we can't even see him any more. So my friends and I shrug and head downstairs, kind of spooked out. We get down there and the door is cracked open and the janitor is inside.
FRIEND: "Whoah, you got down here fast."
JANITOR: "What? I just walked into the building. How'd you guys get in here?"
ME: "The door was unlocked and the lights down the hall were on."
So this visibly confused/upset the janitor, and we didn't understand. But then he asked us which door we came in, and we specified. Turns out, we'd come in the wrong door. The door that we came in is typically only used for deliveries and requires a passcode to enter. The only people who know that passcode are him (as the head janitor), the athletic director and the principal. The other two weren't there, and we had arrived to the school before him.
He goes through his walkie talkie channels trying to reach another janitor just in case someone had already gotten into the building, and then we even went back upstairs to walk around and find the person we'd talked to, but there was nobody in there except for myself, my two friends and the janitor. Creepy as fuck.
I wound up telling this story to our Biology teacher, who had been a teacher there for a few decades, and apparently he said that other teachers and staff and students have had similar creepy encounters/experiences in that same hall/corner of the building that we were in throughout the years. He said that there are several different urban legends that have kind of grown around it to explain it over the years, but he didn't really know much of the history behind it, other than that we definitely weren't the first people to experience something weird up there.
So yeah. That happened.