In S5E1, Castiel places Enochian sigils on Sam & Dean's ribs. He says:
"They'll hide you from every angel in creation, including Lucifer."
However, this appears to be not true in S6E15.
- Balthazar appears uncalled to the brothers, in Bobby's house.
- Virgil follows the brothers to the alternate universe.
- Castiel and Balthazar find the brothers rather quickly after their return to reality.
There may be other, similar incidents I either haven't witnessed or thought of. Is there any good explanation for all of this?
I should note that I've got theories for the latter two, but I still cannot think of anything to explain the first.
- Virgil may have been able to follow some sort of residual trail left by Balthazar's portal, to arrive at the same location the brothers did in the alternate universe.
- Castiel and Balthazar could have been monitoring Raphael, knowing that he would eventually have to summon his servant back.
Still, this does not answer the first appearance of Balthazar in this episode. When I originally saw him gathering ingredients, I thought he might have gone to Bobby's because that is one of the few places on Earth that might have so many such things in close proximity. But, this seems less likely later on when we learn...
His original plan had been to send Sam & Dean to the alternate reality all along.
Therefore, we know that he was looking for Sam & Dean specifically. The proximity to spellcasting resources was probably just an added bonus.