True. I was just thinking it could also mean that they store that chemical. I jumped to the conclusion of thinking it was a pipe because of the bricks protruding from the wall, but it looks to just be an architectural design choice.

Thanks for the links! I’ve been searching for any sort of clue, but I haven’t had any luck either. I wonder if it’s an older custom sign and the codes have changed through the years? Maybe you’re right about it being odd and has been updated officially, but they just never updated the sign.

I was thinking that too. There’s also a hazardous line right where the explosion happened if you look in google maps. It’s labeled in clockwise order from the top: Red 4, Yellow 1, White W with a line through it, and Blue 3. I heard the “W” means it reacts violently to water, whatever it is.