The 3 Goblins that Hetz rescued with his party stayed in Bhurol and waited until nightfall to escape, where they would have better chances, they saw Hetz gets caught trying to escape during the day, and being taken back to prison, they aim to set him free as a return favor.
The time they were waiting for nightfall wasn't spent in vein, the goblins managed to secure their gear back.
When the sun sets, the goblins sneak into the prison in an attempt to free Hetz.
The door to the cell is jammed and can be force opened with DC 15 strength check, no key can be found, because the prison guards lost it.