ZHP overheating issues

Hello everyone, recently my 330i has started to overheat and I'm not sure why. I have replaced thermostat, radiator, expansion tank and the entire cooling system upfront as far as that goes. As well as fans. My bmw mechanic thinks it may be the lower coolant pipe that is leaking but they are not for sure. There are no visible signs of a leak. I pulled into a parking garage today and noticed the guage creeping out of the operating zone, I was able to turn it off before it left the right tick. The car was just bled aswell.I need to get rid of the car now for sure, it has had too many issues as far as cooling goes. I just need to understand what to fix before I get a 335