@Echonic Wait, didn't they theorize that the original owners of the body might not have actually come back? It's just that in order to understand the girl better and behave exactly like her, destruction achieved such a state of perfection that it basically became her in trying to 100% copy her (destruction ended up giving up the memories and thoughts that make destruction destruction trying to subconsciously become closer to the girl whose emotions ended up as an unaccounted catalyst to that, which caught them off-guard cause no one could've predicted that). If that's the case then she probably didn't come back, destruction just lost itself within her.
But nothing has been proven yet. I just think this theory is a much more interesting plot point than them just investigating and falling in love.