I've been postive with the Legion for a long time and a Frumentarius didn't show up, so I thought I would try going to Unpredictable and I still haven't seen a single disguised Frumentarius. Have I done something wrong?

You have to kill benny at the tops or let him run to the fort and the guy should be outside the tops and give you it

Do you mean for the guy that tells you, that you can have some legion loot. He can turn up at any time while traveling the wastes

You're thinking of Vulpes/Alerio, this is something else.

I'm looking for the guy that tells me where to find the Legion loot.

Oh Ive never encounterd him before

Apparently this quest can bug and be deleted and you never see the quest giver. Even with a mod I've got that fixes this problem the Frumentari can end up getting killed, weird thing is if this happens his corpse follows you around when you fast travel. It's very hard to spot though because it's usually half buried under the ground. I've only ever got this quest given when my reputation has been mixed.