Why did Camp Jupiter have a sign saying Hades? [hoo]

In Son of Neptune when Percy reaches camp Jupiter, it said there was a sign pointing straight down saying Hades. Is this just an oversight or was there a reason for this?

Additionally, after Mars appears and Percy said they fought he said something along the lines of "Not as Mars, you would not be here if you did". Did Mars/Ares know about Heras plan at that point? It took place a few months after The Lost Hero but I'm unsure if Hera told the gods and it sounds very risky to just reveal you have multiple forms like that.