Nice!
The plug looks like the ones for the fog lamps. See if you have one on each side not connected to the fog/driving lights. Do you know if your 318ti is a standard or an Active model? It could also be the connector for the alarm horn, being on the front passenger side.
Check coolant level over the next few days, but as you probably know, not when it's hot. OK. to add to it when it's cool to the Kalt or cool level.
Don't know of a coolant return off the expansion tank. I believe the cap is set to 2.0 BAR if it over pressurizes or too full. I like the BMW coolant as it will dry white so it's easy to see areas that leak or overflow.
Probably says it in the DIY, but take care not to overtighten the bleed screw on the top of the radiator.expansion tank. It can strip easily.
So, as a few have mentioned, the other parts of the cooling system are ones to watch or predict as the weak links after the amount of mileage. Water pump and hoses are typically next for me. the two plastic connectors can be problematic as they get brittle and can break. Perhaps they were replaced 70k ago if the engine was replaced, but maybe not. the hoses and 1 of the connectors are easy to change. The rear one on the M44 is a challenge and if your "Golden hands" happen to be small, it may be an easier job for you. My paws are too big to make it an easy job.
Anyway, GREAT job on the replacement. You're off to a good start!
Cheers!
