Lily knew her best friend is hiding something from her - there was no way someone who was on the brink of death and was bedridden for days to be suddenly up and about as if nothing had happened. Not to mention, that strange symbol and wings she saw in the hospital wing. He definitely is hiding a secret from her.

Severus Snape does not exist. That character merely a creation by a demon who wanted to blend in amongst humans to ease his boredom - that was until he got bored of his mortal life. [Marauders Era]