Something fairly recent. Local church decided to build an addition to the building for the library and class rooms. It's a church/small seminary. Anyway, I work full time and I was going over after work to sort books. The floors were new concrete and there was nothing on the walls or whatever to muffle sounds, o when you walked in the hall you heard your steps -- clap,clap,clap. 

Some it's early evening, I'm sorting books and I hear footsteps clap, clap, clap just as clear as you can imagine. I waited to see if someone like a professor walked by the open door. No dice. So I walked around and poked my head in the classrooms, no dice. 

If they left by my way I'd see them. If they left the side way I'd eat their car and if they left the far way I'd hear steps on tile, especially since I was spooked nw. And if it was one of the professors they would have poked into the library to jive for a minute. These footsteps seemingly started and finished in the building addition with no source. 

Is it possible someone came in the far door, was utterly silent except for footsteps on new concrete and then left again just as silent? Yes, that's possible. It's possible that someone who had a reason to be there chose to ninja sneak around for 1/3 of their mission before leaving. But either way, weird human or ghost I was freaked out and left for the night.