That's a good point. It is hard to say if he is still in the yellow power battery or not. Since the battery was in a way decommissioned, Mongul could have slipped out. I can't imagine the guardians knowing about Mongul being in there. Wherever he is, I imagine it will be brought up. Sooner or later, either Sinestro or Arkillo will be remaking the yellow lantern corps (or whatever they decide to name it). More so if Arkillo revamps the corps, I imagine Mongul will be returning to take claim on the corps, since last time Mongul was able to show his dominance. It is a looming story that I believe will happen, we just need to wait for the current stories to end.

i agree were is he and the yellow battery is what fuels all the yellow rings