On the 02 I think there was a connection you had to make on the master harness near the ECU in order to access the reflashing mode on the ECU. You can always look for that connection near the passenger side floor. I don't think the jumper had to stay in place for the ECU to hold the tune but that doesnt mean people always remove them after their tune.
You can always take it to a respectable tuner and have them do a really conservative pull on their dyno just to be safe. In fact if its got an intake and a turboback it might be worth the money to have it tuned anyway. If ther previous owner had it tuned already then they wont be making a ton of changes but worst case scenerio if it didnt have a tune your tuner could extract some nice power out of the turboback.
13 WRX Hatch - Torqued Performance Stage 1+
Slow is Smooth > Smooth is Fast