(Okrand, qepHom 2019):  The difference between {mej} “leave” and {tlheD} “depart” is that {tlheD} implies setting out on a journey, having a goal or destination in mind, while {mej} refers simply to leaving one's current location. In both cases, the object is the place you're departing/leaving from.

And let's remember that the difference is not whether the subject has any destination in mind; it's whether the speaker is expressing that the subject has a destination in mind.

ghIq mejpu' tlha'a HoD. Then Captain Klaa left. (It doesn't matter whether he had somewhere to go; all I care about is that he wasn't here anymore.)

ghIq tlheDpu' tlha'a HoD. Then Captain Klaa departed. (He had somewhere else to be.)

-- 
SuStel
http://trimboli.name