I just noticed this barrel in my garden outside..does it do anything?

It's from the Thieve's Guild. I think you get it after you re-establish a footing in the hold.

I've also had 'em pop up before doing those quests. I think it's a glitch.

I don't think it is, I always conidered it to be a friendly stash for fellow thieves, once you're in the guild.