We use this expression to introduce something we had not thought of before.
You can use this phrase in ………… language.
A: Have you seen Petra anywhere?
B: No. Come to think of it, I haven’t seen her all day. Is she here at all?
A: I don’t know.
A: It’s a strange town. I haven’t seen any bars here.
B: Come to think of it, I haven’t seen any hotels or restaurants, either!