Big end failure isn't unheard of on 2.5sti's so it'd be plausible for a similar failure to happen an n/a outback.
The lack of any oil under or in the engine is pretty puzzling tbh though. Could it have possibly been burning oil for the last good while? If it was leaking a drop I'm assuming you were having to keep it topped up? this might have hidden any oil burning problems as naturally the leak would be the prime suspect.
How much driving had been done since the work was done?