Indeed, the entrance was hidden by normal trees in the south. It contained another honey-tree vault.
This must be my lucky game. The Trollshaws gave me 5 honey tree vaults with nice loot and all.
I cleared the first honeytree vault with my arcane blade only at character level 2, so with even less HP.
It's a simple manner of outrunning the bears when you need healing and killing them when you don't, with some patience.
After defeating Bill, and selling stuff at Berth, I made a stunning total of 128 goldpieces thanks to those vaults

.