[Pitsparky]

So no leaks underneath? Sounds like a possible head gasket failure and as it's been overheated numerous times now so the head will likely be warped. It's unfortunately likely already too late but you shouldn't continue to run it in this state, worse thing you could do. Might be a leak via the egr or similar but you need to get it diagnosed properly by a competent person. If it's head gasket a replacement engine might be an option. But you don't run modern engines if they're overheating, not for more than minutes, they will self destruct.

 

[Pitsparky]

The engine still feels great and no signs of milky residue.

I've not driven it today and the coolant is more full than yesterday(well above the fill line). When I opened the cap coolant was leaking, but not from the cap. This doesn't make sense to me. Could this be a clue to the issue?

Milky residue us just a sign and certainly isn’t always present as oil doesn’t necessarily get into the water with a blown head gasket/damaged head, thec rusty water is a bad sign too. Engine fine? Unfortunately at the moment it certainly isn’t and just because it starts and drives doesn’t mean a lot until you find the (possibly very serious) cause and any damage sorted. Sorry for being negative but your continued use in that condition is usually fatal for the engine. Never continue to run an overheating engine ever. If the heater (only got one?) doesn’t work that tells you straight away the water is low. If a car is overheating you stop and check the water level immediately before damage is done. If the oil is overheating that tells you the oil is picking up the excessive heat throughout the engine and no water is available for the oil cooler.

Leaking when you remove the water cap, where from ???