My car had exactly the same symptom where it leaked initially then stopped. However one hot day coolant started leaking out far worse than before, which why I towed my car to the Z shop to patch the leak.
Where was the leak in the end? Since flushing my coolant system my car has been fine. Not leaked anything. I have a feeling it might have been the heater matrix bypass pipe as I took it off to look down the back of the engine and since putting it back on its all good. I did however get tons of rubbish out of the coolant. I took the drain plug out of the bottom of the radiator and nothing flowed out untill I took a screwdriver and pushed it in the hole. I got lots of what looked like sand out of the cooling system I ran water through the whole system for ages untill it ran clean and since topping it up with the correct mixture its been fine. Beavis
Its always easiest to look for the leak when its leaking, taking care of course that the coolant will be hot. As stated, it helps to see the leak if the coolant is green. You can tape some very small pieces of paper (ideally litmus) around the areas you suspect to narrow it down, if they become wet the water will discolour them and help pinpoint the area, other than that you have to wait for the leak again. Based on what you said: One would assume that it was leaking at the time as it was very hot or wasnt relieving built up pressure (radiator cap sticking or with a high psi rating) or is a hose that is cracked and when it moves it leaks Sounds like the hot water heater system tee offs given the location, but could be the turbo inlet/ outlet lines as they can be hard to see at best. Theres not much else there given you did the plenum cooling delete (doing the plenum delete can add more pressure to the tee off return than before, so its a good place to keep an eye on) I also agree with techtraders comments on oem radiators etc All you can do now is wait for it to return, shouldn't be long based on what you said.
