The recording of Mothman's voice in the last episode was definitely eerie and probably one of the more creepy moments to come out of the show in a while, IMO. I believe the Mothman is real and he's someone (or something) I definitely would never want to encounter and have respect for, unlike the guy in the episode who antagonized him further with the way he tried to warn him away. I kept thinking he's only making it worse with that attitude.
The whole "I am here" thing is BS. First of all, why would that bunker be haunted? It was a munitions bunker during World War II in West Virginia - no battles were fought in West Virginia (or anywhere in the lower 48), and the bunker didn't accidentally explode killing soldiers guarding it, or anything like that. So no real reason for it to be haunted. Yes it is an eerie place, because it is dark, dank, windowless concrete bunker deep in the woods. I get it.* It is spooky and the air is heavy (because it is stale, and full of mold and dust) and when in the pitch black in spooky places peoples' senses are heightened, but they are also on edge because they believe a ghost is around the corner. These people hyped themselves up and went into this old creepy bunker for the sole purpose of finding ghosts or Mothman. No other reason. Same thing with them being in Point Pleasant. I am sorry, no one goes to some random little town in BFE-know where just because they can. Point Pleasant is known for Mothman, it is their tourist trap (kind of like Roswell New Mexico has alien themed stuff). The Mothman legend is famous, especially in West Va / Mid Atlantic area. They were there specifically to look into Mothman. So they already had the power of suggestion and are trying to find this thing. Anyway, back to your topic, the man could have easily changed / deepened his voice and say "I am here!". Easy peasy. Maybe they just wanted to be on the show? Also, if the Mothman was flinging his demonic bird poop at them in their house, you'd think they would take some of it as a sample and try to get it evaluated! Who knows. I mean it was interesting story, different than the usual ones plus it was about a cool urban legend, but it still seemed a little hokey to me.
* I live near some old bunkers too (although in a big park, not deep in the woods, but they are large bunkers) - but those were built before WW1 (as a defense of DC area) and in WWII was used POW camp for Germans - one of whom was killed while trying to escape. I climb on those bunkers all the time, although they have locked gates baring the entry ways now, but when I was a kid they didn't and we used to go exploring. It was creepy as hell, and of course we thought it was haunted. But never experienced anything besides feeling creeped out and feeling like being watched and possibly seeing things out of the corner of our eyes. But I think that was all power of suggestion.
I mean the Mothman legend is cool, but it could all just be power of suggestion and a big freaking owl! I felt like while these people tried to sound like skeptics, the fact that they drove 4 hours to Point Pleasant West VA (which is not known for anything else other than Mothman and that bridge collapsing - which Mothman supposedly preceded as an omen) proves that they were there specifically to hunt down Mothman - which makes them biased / under power of suggestion. Also, yes old abandoned bunkers are creepy, but that doesn't mean they are haunted! And all of the satanic graffiti on the inside of the bunker doesn't mean a demon / mothman lives there, it means some local teenagers were bored and decide to scare each other! The bunkers near where I live were covered with that sort of graffiti on the insides, that is one of the reasons the park police put gates with heavy locks on all entryways! To stop stupid bored people from destroying history. It was an okay episode, interesting premise, but the whole thing is fake.
The owl skeptic theory is pretty insulting. The place is a rural area. I doubt people would be mistaking an owl rare or not for whatever they thought they saw. It's just a skeptic dismissing eyewitness as dummies with a lazy explanation which if it were true we would have seen Mothman sightings for ages all over the country and many other countries.
Your comment involving the "power of perception" made me wonder if perhaps the Mothman is a tulpa?.. I haven't looked into this at all, the idea just popped in my head.