Well here's what happened to me. I had 2 experiences in the same building a few weeks apart. I have told them before on MN.
I should say i don't believe in 'ghosts' and i really don't want to believe in anything else to be honest!
So the first thing was I had a tea cup fall onto the floor out of nowhere right behind me (it smashed). I was at work - a collection of buildings - some old, some not. The part of it i was in at the time was not.
I was the only person in that particular building at the time, i needed the loo and had walked down an empty corridor and had just arrived at the loo door, which stood open. I was perfectly happy, going home soon. Just before i stepped through the door into the loo cubicle the cup smashed right behind my feet in the corridor.
I looked around obvs. Corridor empty 
I had to pick up the bits, and put it in the bin. It was clean, a plain pale green old fashioned style tea cup. I'd never seen this cup in the staff room.
Weirdly in the moment i wasn't overly spooked by it. There was no feeling of malice or dread in the air. I mean it was sunny outside, ground floor, big windows and it was a bright clean modern building. I just thought it was bloody odd!
A few weeks later i was walking down the same corridor but a bit further along. Alone in the building again. Sunny again. About to go home again. I walked past an 'open to the corridor' carpeted area on my right, picked my phone up from a desk round the corner and walked back again. Didn't take much longer than it took you to read this paragraph.
As i had passed the carpeted bit the first time i had noticed some bits and pieces which had been kicked under a coat rack and made a mental note i'd pick them up the next morning. Walking back past the carpeted area i glanced to my left and there in the middle of the floor were all the bits that had been under the coat rack. They were balanced perfectly one on top of the other in a 'tower'. About 3 foot high i guess.
I have zero explanation. I didn't touch it, i stopped and looked and then carried on walking! I only started to freak out a bit after i'd left the building. Delayed reaction.
I didn't tell anyone at work for obvious reasons, but couple of days later someone who works in that carpeted room said a weird thing had happened to her - a clock had fallen off the wall above her desk and instead of landing on the desk had sort of shot into the middle of the carpet area about 10 feet away. Same spot where the tower had appeared. She said it didn't bounce off the desk - just flew off the wall!
I still never said anything about what had happened to me. I have in fact only told 3 or 4 people in my life because it's all just too - weird and unsettling :(