I guess this should instead be a blog post given the personal nature of the post, but who really uses that feature anymore?
Devin Weston's Ghost has always been a rather interesting myth to me for no particular reason. I've investigated it both before and after the page was created on the wiki, before and after 100 percent completion, and with all three protagonists. I've put in just about 3 or so hours into hunting down this particular myth, which for such a minor thing is enough for me to give it a final judgement.
The post will be rather disorganized, since I'm just going to look at specific facts related to the myth and I think of them.
Of course, I haven't exactly had much success in actually hunting down this myth, but in my time at the scene there's been a small amount of evidence that could potentially mean something is there:
On two occasions, I have seen props in the area move on their own. Both times were on the upper deck area and were chairs. One time was a large black chair, and another was just a normal lawn chair. In both instances, the chairs were yanked backwards by about a meter and then tipped over backwards. While this is quite strange, it's entirely possible that some sort of thing could be in the game naturally, perhaps to emulate wind.
My next piece of evidence happened today, and it's rather extraordinary. I was playing as Trevor hanging around the black chairs, which I had seen being moved on their own in a previous investigation, going north (which means I wouldn't be facing any lights.) When I shifted my camera to the right, I saw a white orb directly above the table which promptly disappeared moments later. Could this be the force that moved the chairs in earlier investigations?
While investigating around Devin's mansion, it becomes apparent that the area was really poorly optimized. For example, when I would go past a corner, there would be a split second delay before certain props would appear. This was most prominent with the white umbrella model, which is quite ironic given that it's appearance is similar to the classic bed sheet ghost cliche, possibly acting as an explanations for some sightings of the ghost. Additionally, the black chairs I mentioned earlier are very peculiar. I noted a glitch that the chairs would fade in and out, as all props do at a distance, simply by traveling from one side of the deck to another. The effect creates a black transparent blob, which might catch unaware myth hunters. Not only that, but if the chairs are tipped over (by a human or otherwise...) the chair's bottom texture is extremely glitchy and either shows as completely transparent, or a black mass of random lines.
There is a huge amount of lights at Devin's mansion, and they come in a wide array of colors too, this creates a lot of problems for myth hunting. It's also probably the cause for the area's extreme optimization, especially on a console port of the game. One thing I've noted is that, at night, some rooms are lit up. For realism's sake, these rooms emit some of their light outside. It seems Rockstar got this effect by creating a very weak light source right on the wall (using the window texture) itself. This can give the impression of lights being in a place where there is no logical source of it, and in some cases, a small lens flare is seen when looking at the window from certain angles, another factor which could easily confuse the player.
This may be the strangest part of the whole mystery. At night, intermittently, I would hear a very strange droning or humming noise around Devin's mansion. In my investigations, I would first hear the drone noise for a matter of seconds, which would be considered as probably just an airplane flying overhead or another noise. As I stayed in the area longer and longer, the humming noise would stay for longer periods of time, eventually being always there at approximately the hours of midnight to 3 AM. What this noise even is, is quite hard to describe.
When trying to find the source of the hum, I have a few theories:
1.) I've determined that the literal source of the hum, as it would be in the game's most technical sense, comes from somewhere in the middle of Devin's house. This is because the closer I got to the walls, the sound intensifies, the farther away from the walls, the sound degenerates. It becomes inaudible by the time the player reaches the front gate outside.
2.) There are two air vents on the far west side of the house, one for each floor. The bottom floor vent is rather large, but the top floor vent is about the size of the player's head. It's possible that this is where the sound originates from. However, no smoke or anything is seen coming from these vents, which doesn't imply that they are being used. Additionally, the behavior that I've mentioned above doesn't really match that of the vents - the sound doesn't originate from them.
3.) On Devin's roof is a strange looking satellite dish. It kind of looks like a snare drum on it's side rather than an a true satellite. It may be possible that some sort of malfunction of the dish is emitting the sound, but I admit that it seems incredibly far fetched.
So there's my ramblings about Devin's mansion and all the weird things I have spotted while hanging out there. Hopefully it can provide some kind of assistance to anyone looking to check out the area themselves, even if it is mostly theories. Let's look forward to solving the strange droning noises, especially.
Moving props are most likely to be PS4 glitches. No matter what, consoles still have many bugs in them. I, personally, never noticed any prop moving on its own there.
However, when I chose to investigate it, I found a strange site, which could be someone's grave. After I done a face-plant with Franklin, it was just slightly longer than him.
Moreover, when I went to the lower deck, I found a girl sunbathing on the chair and a guy sitting by the pool. When I decided to go back, there were another two people, a man and a woman, both sunbathing. However, the guy went up and pushed me, resulting in his death by stabbing, while the gal jumped over the glass fence-ish thing, nowhere to be found.
As to the hum... only weird sounds I heard came from the traffic and aircraft. It is also likely that the hum was your console (I don't own a PS4, so I don't know what sounds it emits so...)