Secret passage out of Harbor City.

Is there another way to discover it, other than completing 4/4 quests for the Don?

Yep. If you know where it is you don't have to do any quests to use it. At least it did that for me. Very very early in the game when I arrived at Harbor City I was stealing everything I could carry off and found the hidden passage by accident. Used it whenever I wanted.

Also the passage is a quest you can do "For the Order"

There are actually two ways in to the passage, both via secret doors. But then you've got to get out of the passage to the outdoors and that requires a key.

If all you want to do is get out of town and you have an acrobatics ring (I really should get trained in that, but I've always got an acro ring on so nobody ever offers it), you can jump off the walls. The west wall has a ton of guards but it can be done. The east wall is much more convenient, IMHO. Here's a quick video showing how to get out. I don't know if you have to pay again to get back in.

I already know that you don't actually have to be told about it to activate it, I was just curious who else could tell me about it. I don't plan on joining either the Don or the Order.

Ooops, now that I think about it, that second way in only works after you found the first way in.

I found it by figuring out where the Don's men were going after I kicked them out of town.

Originally Posted by Zloth
There are actually two ways in to the passage, both via secret doors. But then you've got to get out of the passage to the outdoors and that requires a key.

If all you want to do is get out of town and you have an acrobatics ring (I really should get trained in that, but I've always got an acro ring on so nobody ever offers it), you can jump off the walls. The west wall has a ton of guards but it can be done. The east wall is much more convenient, IMHO. Here's a quick video showing how to get out. I don't know if you have to pay again to get back in.

There is a much less hazardous way over the wall than the way in the video. Leave Commander Carlos's place and angle to the right. Hop up on the wall and follow it to the wall corner. Look to the right and jump down on the rock, then down to the ground. To get back in go to the south gate and talk to Leto….don't pay him again.