So, i've been taking a look to a few discussions saying "oMG tHerEz a gHOzt in dA tuNneL!!!111 OMG sO sCarY" which is fake. There's 2 explanations for this:

1. Kenopsia: Which is the unsettling atmosphere of an empty place that makes you feel observed. It's a normal thing that happens to everyone but you don't have to ♥♥♥♥♥♥♥ exaggerate it.

2. Malicious mods: Another reason for this is some kind of malicious file in one of your addons which could contain a recolored design of the old gordon freeman or something.

Conclusion: It's either one of these two reasons. You don't have to be around telling people that you found ghosts and try to scare people. It's not worth it.