For those of you unaware, (and because I have yet to make my post), unless you know about it, you can't find it. The passage is hidden with magic. Such passageway was made because when the fortress was built, the land was in a distrust situation. The commanding officer (at the time) didn't have much faith in the castle holding out a siege war (He didn't have much faith), so he built a secret passage way that leads out.
My guess is that the passage way leads to a river or somewhere where he thinks he has a safe passage out of this place. By hiding in the river and traveling with it, one might be safe. Even if there are patrols, you can easily hide by diving in the river. There are many kinds of magic as well as alchemy recipes that allows the user to breath underwater during a short duration.
However, if the passageway is hidden with magic, then it could be tricky to open it. I think using Acan to open such passageway (if it's hidden with magic) is the way to go. Since Acan is skilled in disrupting magic, he could theoretically disrupt without the mages noticing.
Again, those are just some idead I have
Since Acan, you have yet to post yours, you could suggest the idea of it being protected by magic and that you should be able to disrupt it or something.