When Gorion tells you to prepare to leave Candlekeep, why doesn't he do the same to Imoen? Reading the note from Elminster to Gorion, it's obvious they know she's Bhaalspawn also ("those in your care" being plural), so why would Gorion choose to save only you?

Maybe he knows she'll follow after, but that seems like way too big a gamble.

Imoen is annoying. Obviously Gorion thought so too and tried to ditch her.

Perhaps Sarevok only knew of your existence and Imoen wasn't in danger, hence her not having to leave.

Seconded.