When his girlfriend goes missing, David must track down her whereabouts after he realizes she?s not who she was pretending to be.

Not sure if I understand the premise. So David realizes his girlfriend is pretending to be someone else? Instead of drawing me in, it just kind of confuses me. How about something like: David must track down his girlfriend's whereabouts after realizing she?s leading a triple life.

A few little notes:

First; I would start with the word "After" instead of "when." Not a big thing but it will help the logline.

Second; (I have said this before) If you tell us a character "Must" do something, then you have to give us an "or else" otherwise the logline will seem incomplete.

Finally; if you tell us "he realizes she?s not who she was pretending to be." It suggests other (more dangerous) people are searching for her well.

That would give you an "Antagonist." for your logline, Someone dangerous who's standing in the way of your protagonist achieving his goal.

Hope that helped, good luck with this!

Oops sorry I meant comment for the tag line before you shortened it! I like this version a lot. Cheers