First, some background:
My wife and I are young, just out of college, and we currently live in a classy apartment in an older complex (early 70s) in AZ. Our apartment is on the third floor. We have had some strange experiences with things dissapearing and waking up at night with an uneasy feeling that someone was there.

And now, the experience:
Last night, I woke up after having a series of bizarre and uncomfortable dreams. I thought that it was a little bit warm in the bedroom, so I grabbed the remote control for our oscillating fan. I couldn't quite remember which button set the speed, and it was too dark to see, so I took it with me when I realized I needed to visit the restroom.
In the restroom with the light on, I looked at the remote control to find the button for the speed control. The fan is fairly expensive and complicated, as is it's remote control, and it includes a thermostat on an LCD display, along with the speed and oscillation settings.
Content that I knew how to use the remote, I left the restroom and walked passed the entrance to the hallway that opens into the family room and felt a terrible feeling of forboding... like I was expecting to see something right there. I remembered that the remote control I was carrying had a thermostat and the hallway was lit just well enough (thanks to the light from a streetlight coming in through the kitchen window) to see the display on the remote. I looked and could watch the display counting down... I watched it count all the way from 75degrees to 67degrees before I got too creeped out and ran back to bed.

I know it sounds trivial, but to me it means something. I've been suspecting that there is something strange going on in my apartment for some time now, and this is the first "evidence" that I've produced.

Just wanted to know what you folks think...

Wow, my girlfriend and I had a similar experience last night as well! We live in an apartment in Florida and usually we keep the indoor temp at 75 degrees. Well, we both have had strange sensations the past month. It ranges from a weird presence looking over us while we are sleeping to seeing what everyone calls orbs out of the corner of our eyes. I won’t say anything in order not to scare her, but the weird thing about it all is she will advise me that she just saw an orb float by out of the corner of her eye at the same time I saw the thing. There are no strange light sources and we will both feel a presence at the same time...

Well onto last night, as I mentioned, we keep the place at 75 degrees. Well, she felt the need to put on a sweater because she was freezing, but she is always cold so I didn’t think anything of it... But she was so cold she had to go out onto the porch and sit where it was very warm and humid. Needless to say I had to tease her about it because It's freakin 75 degrees! Well, later on when we hit the hay I got up out of bed to get a glass of water and as soon as I walked into the hallway, I started to freeze! It was the same cold as Green Bay WI in January! I was freaking out... I walked out of the room to see what the house temp was and it indicated 75 degrees (I don’t have an external thermometer) and there is not a window in the hallway either (no drafts and its humid & warm here anyways). Once I walked into the bedroom I was fine but that was weird! I've never experienced anything that bizarre!

I should add that we occasionally go on some amatuer ghost hunts and love to stay at various so called "haunted" bed & breakfasts in the US and Europe. While doing this, we have both experienced some unexplainable things... and I am still of the opinion that "ghosts" are nothing more than an energy residue remaining in an environment stemming from some tragic or extreme event... but what happened last night and throughout the past month has been quite odd and whats even more funny is that we venture off to attempt to experience such things! I'm sure it's nothing more that an overactive imagination but it is still weird that we both have sensed & viewed the same things at the same time... even when we dont talk about it for a few days or so after the initial occurrence...

It happened to me once. But you probably pressed it by accident or something. Once I left on the fan on and after 1 hour it automatically switched off... I know its not the electricity because my alarm clock was on.

It must have been strange to see the thermostat drop like that in arizona. that is probably a state record low. Is there a landlord or anyone who could tell you some history of the appartment's previous tennants? If you think you could have a haunting, this could provide some evidence you need to solve the puzzle. But you could also check to see if there is any work going on to the buildings heating/air conditioning at the time. In fact, I would ask that question first just so your landlord doesnt think your a wacko. Some people just don't understand

If your a/c was off, and your windows open, you could have a draw of a/c air coming in from an adjoining apartment via the duct, exiting through a vent in the area you observed the tempature variance. When I lived in the South I has my a'c going 24/7, perhaps a neighbor of your does the same- and this is creating a forced air entry in your dwelling-
I just had work done on my home, where all the vent pipes exit on the roof I had the carpenter cover with a neat little roof and stoned sides, this action totally changed the air pressure in my home( I had a massive cold air leak coming through, from behind the kitchen cabinets, did not have prior to repairs on roof.)
Maybe this will help, just a few thoughts-

I often wonder why the presence of a ghost is often accompanied by a drop in temperature.

First of all you're in Arizona.Secondly your apt maybe on an ancient burial ground.Since that maybe a possiblity you could be having a brave visit you,and desiring you to leave his resting place.You may not be the only ones there having this kind of phenomenon taking place around you.