It's a secret message. You got to look at it at least twice - once in the morning (presumably daylight) and once in the evening (presumably dark). The rest of the message on the billboard only shows (glows) in the dark. But the part that you see in the morning (and of which you took a picture) doesn't show (glow) in the dark. And by looking at it twice - in daylight, and in the dark, you get to combine the two messages into one and that one is the secret message. :-) How about this explanation? Would you return to look at it in the dark? And let us know? :-)