Paper planes is a state of being. In reference to the MIA song it is the state of being missing in action, or leaving a place. It can be used both as an action verb and an adjective.
TC: Yo MG hasn't drank with us in a while..
The General: he's paper planes
And dying in yo' bed many years from now, would ya be willing ta trade all da days from dis here day ta dat fo' one chance, just one chance, ta come back here as young men an' tell our enemies dat dey may take our lives but dey will never take our freedom ... YUB NUB!