I have trouble believing that someone at the level of a PI at NIH would just quit with no notice (unless there was some kind of misconduct and they decided to leave rather than get fired). They are running a lab. They direct all the projects that the lab is doing and they are ultimately responsible for all the grant funding. A PI quitting without notice would be incredibly disruptive to everyone else in the lab. I'm not sure how it works with NIH intramural research, but at a university or a hospital you'd have to figure out how to transfer any grants, what to do with equipment paid for by the grants, and what will happen to the grad students, postdocs, and everyone else in the lab. It would be incredibly irresponsible to leave suddenly without addressing those things.