I could swear I've seen rules for Minions in DH , but now I can't find them. Am I missing them, or am I just thinking of a mechanic from a different game?

All I can think of is the BC Minions system (and comrades as a lesser extent). I remember there was some mention of minion rules for enemy NPCs, where they have 1-3 wounds and the PCs can take them down quickly, rather than get bogged down in critical hit tables.

Inquisitor's Handbook also had a bit about services you could hire, including a bodyguard I think, but it was very rudimentary.

What exactly are you looking for?

My players are high Rank enough that, if they were D&D characters, they would start accumulating Henchmen right about now. For some reason I had it in my head that there was a similar mechanic in DH ...

Well, I'd say the BC mechanics mentioned by Cymbel sound like exactly what you need. They should be fairly easy to implement in DH as well!

The rules for minions are in the Creature's Anathema in page 134, basically they die instantly when they take one wound after toughness and armor. You can also give a minion multiple wounds, but they only lose one wound at a time regardless of the damage dealt to them.

A more detailed approach if you want character to their minions, is give them new characters. Have them role up a new sheet and detail them as lackys. As the GM you can roleplay them, or just give them over the the players themselves. I found it more fun to have the other members of the group roleplay the other's minions. To help encourage a separate mindset.