In semiotics, a sign consists of the "signifier", the symbol or word that we see and the "signified", the meaning of the sign.  At the Turner Valley Gas Plant National Historic Site, not far from Calgary, Canada, the doors are marked by enigmatic circular symbols. We know the symbols signified hazards within, but careful inquiry has so far failed to reveal the specific hazards that the color, size and placement of the signs denotes. Hopefully, the meaning of these signifiers will not be lost for ever.
As I explored the symbols, I could feel through the brush strokes and spray patterns the presence of the original painters,  the co-creators of these modernist images.