In the Genki textbook, it is said that "In the casual conversational use of short forms, question sentences do not end with the question particle か, but with rising intonation alone."
Nonetheless, in the manga I'm reading, it's written 「そろそろいくか」 (meaning "I should get going" right?).
My thoughts here is that Genki didn't explain the whole context, as I've also heard this same expression in animes. Could someone clear it up?