Jason is a demigod, specifically son of the Roman god Jupiter. It's canon that a lot of monsters/creatures/gods can sense/smell this about demigods...apparently it's a very pleasant smell to them since most of the monsters are attracted to the smell and want to eat them.

He'd come across as probably pretty powerful too (it is also commented on in canon that children of The Big Three -- Hades, Poseidon, and Zeus -- are particularly potent). There wouldn't be a sense that he's evil, he's actually very much a hero, but definitely not pure human. Since he's the son of Jupiter I like to think part of it comes across like a storm/lightning approaching.

Jason wouldn't be able to sense anything about her demon essence though.