I'm assuming that an unused building would not be well lit.

One class of things that might be mistaken for ghosts in such a building is various animals. A family of raccoons moving farther into the darkness to hide when a human opens the door, for example. Or a lot of rats or mice.

Another would be humans who are clandestinely using the space.

A trail of moving water running down a wall from a water leak might reflect light in such a way as to appear to be some sort of shimmering ethereal thing moving around.

Just to add to other things people have mentioned, reports of ghosts could very easily be the result of false memories, as well. Finding a dead body in a creepy location, then sitting on it for two years is plenty of time and fuel for false memories. (Which are notoriously easy to form.)

Another thing I think is the most likely is [pareidolia](https://en.wikipedia.org/wiki/Pareidolia). Our minds evolved to be sensitive to seeing faces/hearing voices, and so we tend to see these things where they don't actually exist. (Think faces in toast, or listening for satanic messages when playing music backwards.)

Ghosts? In Berkeley? BOOOOOOO!!!

http://www.weirdca.com/location.php?location=322

Cows? In Berkeley? MOOOOOOO!!!

https://www.youtube.com/watch?v=10JSPdTDFsI

https://eastbayexpress.com/mister-dairy-goodness-1/

Animals, humans that aren't supposed to be there, water leaks, carbon monoxide or other dangerous gases.

Carbon monoxide. https://www.ted.com/talks/carrie_poppy_a_scientific_approach...

Perhaps infrasound, described in Vic Tandy’s A Ghost in the Machine

Here’s a brief of it: https://higgs.ph.ed.ac.uk/outreach/higgshalloween-2021/haunt...

The full story is a PDF that is the first google result of “Vic Tandy’s Ghost in the Machine”

e.g. failing mechanical equipment