I don't know what they saw.
I saw something once, though. We were sleeping in the old (unused) hospital building on Fort Collins. I had been sick with a stomach virus. I woke up and walked out into the hall and saw something moving in what was probably a ward room at one point. It was roughly human sized and shaped, but it's features were indistinct. I noticed, while looking directly at it, that I couldn't hear it's foot steps and thought that was odd.
After it vanished from my view, I checked the other guys cots and found them all asleep. The door was locked from the inside and we had the only key to that area. I don't know what it was, and have never been able to rule out all possible scientific explanations. It was weird, though.