My sister's partner used to work in a lab where they analyse chemicals and substances. He will never forget his experience for as long as he lives.
Being a non-believer in ghosts, he never found it in the least bit daunting when he was alone in the huge Lab in which he worked in (although being alone was rare).
One evening he was in fact the only person in the entire place. It was completely empty; there was no doubt about it. It was also 9:30 in the evening, this didn’t scare him at all. He had to enter the chemical Lab, so he put on all his whites and covered himself completely so he could enter safely.
The room itself was also completely white. The only thing that stood out was the huge window where he could see into the room next door. He and his friends would always wave through it when they saw each other.
This particular night somebody walked past quite close to the window so he immediately waved. Suddenly he froze! There was nobody in the entire building, so how could this be possible? He immediately left the room; there must be some mistake. He knew he wasn’t seeing things; perhaps there was somebody there after all. He entered the room slowly. There was only one door in the whole room so he was definite to find out who it was. But there was nobody. He searched everywhere. Nobody!
The next morning he asked his work colleague if there was any chance of someone else being in the room. His work colleague simply said, "So you’ve seen him?" He completely froze. "What do you mean him?" he asked. "The man that walked past the window, even though there is simply no possibility of someone being in the building." He heard this and was all too freaked out.
The truth behind the story is that many years before, a man who worked in that very room was lifting heavy boxes down from a high shelf. The entire stack fell and crashed on top of him. He was alone in the whole laboratory; nobody was about to help him. He died and his spirit still lies there.