I used to work with two nurses who had seen ghosts.
One worked in the long term care unit of the hospital. One night she had been walking down the hall at 3 a.m. to administer tube feedings to a couple of residents. She saw someone enter their room ahead of her. When she reached the door, she realized it was CLOSED. As in, the man she had seen had walked right through a CLOSED DOOR. Incidentally, she told me that other staff had also experienced unusual happenings in that particular room - for example, they would be caring for a resident, hear a noise, and look up to see the reflection of a man in the mirror, standing next to them. When they turned to look at him, he was gone.
Another nurse I worked with told me of an experience she had had as a child. The doorbell rang one day, and when she answered it, an older woman was standing there. The woman asked my colleague if her mother was in. My colleague said no, she wasn't, and asked if she could take a message. The woman said no, no message. When my colleague told her mother about the encounter, and described the woman, her mother said to her, "That was your grandmother. She died years ago."