* A custom marking: Someone may have added it themselves. It could be an owner's initials, a nickname for the truck, or part of a decal or sticker.
* A misreading: You might have misread or misremembered the marking. Could it be another similar-sounding word or abbreviation? A picture would be helpful.
* Damage-related: If it's a partial marking from damage, the full inscription may hold the clue.
Without more context or a picture, it's impossible to say for sure what "NOBUS" means on your 2001 Dodge Ram.