My official creepiest moment as a teacher...it happened today!

Ok, so I am a music teacher for first through eighth grade. Today, I left my classroom to go eat lunch in the faculty room. I came back twenty minutes later to put my drink on my desk before I had recess duty...I walked into my room and put my drink on my desk and then I heard this noise that sounded like a guitar being played....and I looked, and there was this GIGANTIC man sitting with his back to me with one of my guitars IN THE DARK, in MY CLASSROOM! So, I ran out because I thought I was SEEING things. On my way out, I literally bumped into the kindergarten teacher and the library teacher and asked them to look in my room and tell me what they saw...so they got really freaked out too because this man was just SITTING there plucking at one string on the guitar in the dark. He didn't even move when I was in the room. So all three of us went in, turned on the lights and HE STILL DIDN'T MOVE, he just ignored us. So the librarian went up to him and looked him in the face and said, 'HI! I'm the librarian, WHO ARE YOU?' And he just ignored her! Then she said, 'Do you even have a child in this school?' And he ignored her! Now, we work in an INCREDIBLY small school where we know all the parents. It turns out this was a pre-schoolers parent, but I am very creeped out that there was a strange man in my room who refused to talk to us, WENT THROUGH MY CLOSET and took out a guitar and NOTHING WAS DONE ABOUT IT! That is MY classroom. When I told the headmaster, he just seemed to act like it was typical for this man, but I don't feel comfortable with what happened or how it was dealt with. I'm frustrated and annoyed because I have ALL of my personal belongings in there, including my flute that costs more than my new car.