Fiona's Lucky Charm

as you progress through her quests through the zone, her caravan advances with more/different riders. its just the maps way to help you remember where you are and how far you have left. it also helps you track the related achievement a little. if yet to see the caravan anywhere other than the wpl/epl border without starting her quests

and in relation to this buff, it becomes available after finding and returning with her first 2 paladin traveling companions tarenar sunstrike and gidwin goldbraids

its actual effect is to cause hidden stash to drop off of mobs which has a small chance to contain Mr. Grubbs

tl:dr - find fiona, do her 2 quests, do the 2 pallys quests, recieve buff, grind for pet

So, I can confirm that the charm does remain even if you log out, so if you get fed up after grinding for an hour and want to quit for a while don't worry. Thing is, after getting Mr. Grubbs I tried right clicking the charm and that does not make it come off, which I am not complaining about, but I do not know if there is a time limit on the charm.

As for grinding, the best area is Zul'Mashar in North-Eastern EPL with all those troll graves. Tons of bodies to loot, especially if you have area spells and can kill lots at once. The respawn is a little slow but you can kill the other trolls in the small area while you wait. It is the best place because there are lots of mobs to kill and minimum travel.

Alas, even with all those mobs I was still there for at least an hour and half, and didn't get very many stashes in all that time. Some people get lucky and get it after two loots but those are rare, happy stories. The best thing to do with an item that is a semi-rare drop is to assume it will take some dedication, decide whether or not you want to spend the time, and then be surprised if you get it early, or not surprised if you're there for 2 days.

Fun thing for me is that I looted 2 stashes from a group of the trolls and opened them one after the other; they both had Mr. Grubbs in them lol. It's too bad they are BOP.

This charm can be obtained after by doing the 2 quests offered by Fiona in the Eastern Plaguelands.
The charm used to get the pet Mr. Grubbs which is found in Hidden Stash that drop from the level 40-45 mobs in the Eastern Plaguelands.