The verb {mej} refers simply to leaving one's current location, while {tlheD} generally implies setting on a journey with a goal or destination in mind.

But that's rather strange, since normal sane people, never leave any location without a goal or destination in mind.

Normal, sane people, don't suddenly get up, leave their house,work,etc without a goal or destination in mind.

Normal, sane people, don't suddenly find themselves in the street, just wandering without a goal or destination in mind.

Sure they do. "I can see you two want to be alone... uh... so, uh... I'll just be heading out... I guess?" That's mej but not tlheD. You're not leaving to go somewhere; you're leaving so you're not here.

"Get out of my face! I don't want to see you anymore!" You're ordering someone to mej, not tlheD. You don't care where they go, so long as they go.

"Closing time! You don't have to go home, but you can't stay here." That's mej, not tlheD. You're being kicked out of the bar, and the bartender is specifically telling you that what your destination is doesn't matter.

"Cindy, your brother is sleepwalking again. He just left the room." The sleepwalker needn't have a destination in mind. That's mej, not tlheD.

Any why are you excluding insane people from the grammar? Maybe an insane person simply walks out of the room and wanders the street for absolutely no reason. That's mej, not tlheD.

-- 
SuStel
http://trimboli.name