Azie was very worried yesterday as she noticed Hero has started a develop another bulge again, so I told her to bring Hero back for a check-up this morning.
Azie sent these photos:
As I was at work this morning, the vet called to explain Hero's condition. It's called "seroma" (thanks to the commenter who gave me the right spelling!). It is the accumulation of fluids due to the surgery. Those parts, where the organs had fallen through and attached themselves (and now the organs had been removed and pushed back where they rightfully belong), had been agitated and are thus producing fluids. It is the body's own reaction to the agitation.
This may cause infection, but Hero is still on antibiotics, so that should be sufficient cover. The vet had advised Azie to apply hot compression on the bulge and monitor it for another week. It should heal by itself.
Azie came to get gloves from me for the purpose of the hot compression.