I noticed this hive had liquid dripping from the entrance hole. Not a good sign! Hives generally have a fair bit brown propolis around the entrance but is usually pretty dry. When you see liquid it’s a sign that the hive has an issue. This particular hive grew weaker over time so was possibly queenless, so defences were low. A normal strong colony of bees would be able to protect themselves against Syrphid Fly. This hive could have completely died out before the Syrphid Fly even arrived.
The Syrphid Fly will lay it’s eggs in gaps in boxes and they will enter the box and destroy it. The whole insides were a big gooey mess with no sign of any live bees.
This is Syrphid Fly Larvae. Once your hive is at this stage then it’s too late to do anything.
Once your hive is at the above stage it’s just best to accept the situation and move on as there’s nothing you can do to save it. Clean the box out by scraping all the goo out. Pour hot water throughout the box to kill any pests and the hot water will dilute the goo.
After leaving for the box to dry out for a while you can re-use the box for your next hive.