I'm glad I could be of some help after so many years x3 I've forgotten so much about the game that I can't even dig around in the project file to find things because I don't know where to start looking anymore... (the game is so old that I didn't think about scalability or properly commenting my code)
Since you've been stuck on this puzzle in the fire area so long, I looked up the solution. The secret in the fire area is near the sign that says "Boiler Room, Do Not Enter":
The "rusted shut" message is the standard message when a door is an actual door object, but it has no destination. I might have kept some in for flavor, but it's also possible that area is meant to lead somewhere, and I just never finished that area. (All 50 Lore are in the game and obtainable, at least - I finished that part during the game's original 3-month jam development, so all secrets added after that are independent of the Lore system, which is why they're a bit thin on those things)
*looks it up*
Oh, the door being rusted shut at the end of that long lava area probably is intentional - there's all these signs warning you not to go through it before finally giving up and just saying "okay, fine, go through the door already" and then it's rusted shut for comedic effect. There's a lore right outside, so there's a proper reward for the trouble, and the room has a much higher ID than any of the other levels in the lava area, so it was added much later... I probably moved the Lore there because they were too easy to find originally.
Swifthand's message doesn't set any flags, so it's only there for the information dump (it points out two actual secret areas). iirc, my intention was that Swifthand lived before the apocalypse (it doesn't quite get across clearly as it is now since anybody could've written graffiti on a signpost at any time, but since the city is surrounded by monsters and a desert, and was thought abandoned before you meet the Shopkeeper, chances are urban exploration isn't a huge thing anymore, at least not in Columbus)
Interestingly, there's two versions of that canal room with layout differences, and the signpost and breakable path only appears in one of them. I've forgotten what the other copy of the level is for. Maybe it's the Epic Whirr flooding cutscene, or maybe there was a redesign at some point and I didn't want to risk losing the original layout if I didn't like how it turned out, so I kept a backup of the level.
Another fun fact: that level is named canal3 internally in the game sources, so you managed to guess the canonical name :P