Descent 2nd Edition Community Rules Reference Guide
Here is the instruction for the objective tokens representing secret passages:
If a hero escorting 1 or more prisoners is adjacent to this secret passage during his turn, that hero may free the prisoners, 1 at a time.
It does not mention that actions are needed to release prisoners. The "1 at a time" is important as it triggers the message and instructs the hero to discard 1 villager token.
The prisoner ducks into the small passage and cowers in the darkness until you can lead the escape out of the dungeon. Discard 1 villager token from your Hero sheet.
In summary, no actions are needed here.