This just happened last week.
My husband's family owned a home for a very long time. Several people were born in the house and quite a few died in the house. To our knowledge, my husband's great-grandparents and grandparents all died in the home. As was common, there were 2 babies born in the home that died shortly after birth. Finally, his aunt's husband died in the home. Aunt Geri was the remaining person to live in the home.
The house was in serious need of upgrades that were beyond her ability. She decided to sell and move to a smaller home. While her nephews were helping her move, my teenage son sat down at the piano and started to play. Everyone stood around a while to listen to him. Then they went back to work, leaving him alone with the piano. While he was playing he felt someone sit down next to him and place their hand on his back. When he turned to look, no one was there.
A few days after the initial move, piano movers came to take the piano and bring it to her new home. They kept complaining about someone calling their names. No one other than the movers and I were in the home. They got the piano into the truck. One worker was securing it. He yelled and we all ran to the back of the truck. He was very upset because he believed one of his co-workers had touched him then ran. They hadn't.
When we reached the new home I stood outside while the movers assessed the doorways, etc. I suddenly felt a hand on my back. I turned but saw no one. The movers did their thing and the piano was settled in its new home. My husband's aunt was paying them while I checked out the piano. I was slowly checking every key. Finally, I sat down to play. I heard Aunt Geri scream and I jumped up. She said she saw someone sitting next to me on the bench. When she yelled he disappeared. She said the piano was her father's and he would always sit next to whomever was playing. She didn't get a good enough look to recognize the person but it reminded her of what her father did.