I was wandering around Megaton, when I decided to put Dogmeat's fetching skills to good use. I asked him to go fetch me some ammo (while still in Megaton) and he immediately wandered of into a guardrail. I shook my head at Fallout 3's amazing [cough] AI mechanics, but did nothing else otherwise.
In the meantime, I entered Craterside Supply (Autosave 1), left again (Autosave 2), and then fast travelled to a different location (Autosave 3), and finally entered a new building (Autosave 4 – automatically deleting my first Autosave).
At this point, I started to wonder where Dogmeat was, as he had not returned. I decided to wait a full 24 hrs (another Autosave), but still he didn't return. I fast-travelled to Vault 101, where he usually waits when you dismiss him, or if he dies and you have the "Puppies" perk (Get a new Dogmeat). I went back to Megaton, waited a bit longer, but still nothing.
Needless to say, the save where I sent him off has long since been deleted. So my only hope of finding Dogmeat is, well, finding him.
So where has he gone?
EDIT: Dogmeat has reappeared, after a few more hours of playing (I had just written him off as glitched out at this point) outside of Craterside Supply. I think that roughs up to about 1 in-game week, with all the fast travelling and waiting involved.
As mentioned previously, I had sent him off to look for ammo, and when he reappeared, I entered into the dialogue with Dogmeat, as if he had returned. This seems like the normal reaction, but a) he didn't return with anything, and b) the length of time he was gone seems excessive.
So to re-iterate the question; is this normal? Did he glitch out, and if not where did he go for so long?
Best Answer
The pathfinding for NPCs in F3 (and in Bethseda's Skyrim) is sometimes a little dodgy... and to be fair, the terrains in these games can be very complicated, especially around elevation changes.
As a result, the programmers have some "cheats" to get friendly NPCs to logical places (you wouldn't want Dogmeat to reappear in the middle of a hostile bandit camp, for example). You may notice, for example, that in Skyrim you can use dragon powers to sprint/dash FAR ahead of a companion and then turn around to see them impossibly running into frame right behind you.
One of the larger-scale cheats is what you noticed -- Dogmeat disappearing for a looong time and only reappearing when you return to a familiar and safe place.