Ok when i was around level 5 i decided to go to the scrapyard and find Dogmeat. I found him successfully and quickly fast travelled to Rivet City. We arrived together and i told him to find some ammo. I CLEARLY saw him enter Rivet City and disappear. While he was in i decided to do some side quests in the mean time. I progressed through a couple of levels and noticed that Dogmeat had never returned.

I started to get worried for Dogmeat, and fast travelled back to Rivet City in search for him. I looked in EVERY room and founnd nothing. I quickly went back to my house in Megaton to see if he was there but he wasnt. At that moment, i convinced myself that Dogmeat was dead. He was proabably mauled by another dog or owned by a deathclaw. He was gone i kept saying to myself, i clearly must have missed to notice when he died. I soon forgot about dogmeat and contuined on with the game. However, at level 15 i looked up and saw that Dogmeat, if lost waits near Vault 101. Excited i searched all around the Vault, even inside but with no luck. Now i was completely convinced that Dogmeat was dead.

At level 19 (after i decided to install ALL DlC content from my friends disk) i decided to vist the Nuka Cola Plant. At that time i hired Charon and we were fighting some Raiders outside.When all of a sudden i saw Dogmeat running in from the left . At that moment i was bewildered. I couldnt believe that he alive? Yet there he was, starring at me with his name clearly on the screen.

Do any of you guys have an explanation for this? Becuase i sure dont

NOTE:

I didnt take the puppies perk (Besides that is level 20 and above)
I didnt revert save after i knew he was dead
I never found his body or his death notice when i was playing
Dogmeat was just as fragile as he was when you first find him
Double check the Vault, My house, Scrapyard and Rivet City