Sometimes while running if you holster your weapon it goes through the body of your character, only if you stand still the weapon will attach where it's supposed to.
And if you are crouching and holster your weapon while crouching and stand up the weapon will be inside the body (especialy with most of the merc outfitts for the male model)

I remember having this glitch with Fallout 3 too but not every time I played, now I load the game and few minutes later it's always like that.

fix anyone? ...guess not