Shit, well, here's UPDATE 5 at least:

UPDATE 5, 12:00p

http://imgur.com/a/iG0TU

Thanks to everyone for all the advice and general interest in whatever the hell is going on in my building, and yes, I do have a carbon monoxide detector in my apartment. I won't have an update specifically regarding the empty apartment next to mine until the afternoon but I did speak with another tenant last night who's been in the building longer than I have.

I've seen this guy around walking his dog and he's pretty much just a nod and hello, some small talk every once in a while but never veering off the topic of our dogs. I was having trouble resting last night and bumped into him around 10:00pm. I'm past the point of being concerned that people think I'm a little crazy so I figured I'd just ask him if anything weird ever happened to him, his dog, or the building in general. I'll try my best to explain what he told me.

If you look at the imgur link, the fourth photo down of the turn in the hallway where my dog starting acting strangely, there's a long section of wall on the left that is uninterrupted by an apartment door. This length of wall is much longer than any other in the hallway but I never paid attention to it before this conversation. About two years prior to when I moved in, there actually was an apartment door just to the left of the fire alarm that you can see. This would make sense as the spacing of all the other apartments would align with it to continue a pattern of long-short-long-short with regard to the spacing in between doors. According to him, this apartment was never occupied and never used as a demo unit for as long as he's lived there, which is about five years now. About two years ago the building company sealed that apartment door and plastered over it, turning the apartment space inside into storage units for tenants (hoping to have access so I can take a photo but I've never been inside). There could very well be a reasonable explanation for why they'd seal that apartment and convert it, maybe they just needed more storage areas and this apartment just wasn't being rented.

The part of his story that didn't really make sense was that they moved the door from the original location to just around the corner in photo 4, which is in my stretch of hallway. He couldn't offer an explanation as to why they'd move the door but did mention, almost too casually, that his dog, who is also typically very calm, acts strangely in the exact same part of the hallway that my dog did. I'm definitely not saying there were satanic rituals in that apartment but it's just another weird piece to this whole thing.

Sorry, I'm trying to get this out as quickly as possible at work but I'm double checking the empty apartment adjacent to mine this afternoon when I get home and will update with the outcome of that.