So, I went shopping and bought around 30 petty soul gems, a few black soul gems, with an assortment of the rest. Each gem was empty.

I give Lydia an enchanted bow and sword which have soul trap, and let her go to town wacking on animals and what not.

So, around 4 wolves later, I check her inventory and she has every soul gem filled with petty souls....

I'm not sure how 4 wolves filled up around 40 soul gems.....

It happens if you fast travel, which is semi-logical (Killed stuff along the way)