Dog next door

Hi everyone! First time posting but need advice on what to do.

To begin, I live in a “quad-plex” (four, separate apartments in one building). Over the summer, we had a lady move in next door with a dog. Our landlord doesn’t allow dogs so we knew our landlord did not know about the dog. However, my roommate and I have no issues with dogs as we have a cat ourselves (which the landlord knew we had). Upon first moving in, the dog barked a lot. My roommate and I wrote this off as the dog was adjusting to the new environment and street noise as we live on a busy street in our neighborhood. No need to complain as we were sure the owner was aware of the noise.

Eventually, the barking subsided and we thought nothing of it. Our upstairs neighbors informed us they were moving out in a month and they adopted a dog as their new location allows animals and they found a dog like they were looking for. The landlord knew, the neighbors also knew, and he made no noise. However, since the upstairs neighbors had to bring their dog down the stairs outside and walk past our neighbors to walk their dog. When this happened, the next door dog would bark at it. Just normal dog behavior, we assumed. Despite us writing it off, the other, fourth neighbors complained to the landlord about the barking. They explained about the barking when first moved in and the reoccurrence. Our landlord called my roommate and I and asked if we had an issue and we said we knew about it and has annoyed us but that’s just basic apartment living sometimes.

The upstairs neighbors have moved out and have been gone for a few weeks now. Despite this, the next door dog has been barking again. For hours on end. It sounds aggressive you walk past (which is needed oftentimes, as our dumpster is in that direction). It’s a very startling experience when walking alone as sometimes my natural instinct takes over and I forget that the dog is inside, not outside. I’m a little scared of dogs, especially large ones like this one.

Over the last two weeks, I’ve noticed this neighbor has not been home in some time (I like to keep note of who is home in my building, in case of emergencies). I assume maybe she is out of town for the holidays and has someone dropping in to feed and walk the dog. Since I am now out of school, I’m home most of the time. The dog has barked nonstop.

Posting here because it has become disruptive to me. My roommate is with family for the break so she hasn’t witnessed this. None of my neighbors are home also. Should I contact the landlord? Or leave a note for the neighbor she can read upon her return (I do not have her phone number)? I’m honestly concerned for the dog since he sounds to be in distress as well as the disruption to my sleep at night.