I've had good luck finding odd hose leaks by pressurizing the coolant system. Warm up the car, turn off engine, pull out the overflow hose from the tank and put about 10-15psi into it. Ive found failing hose clamps around the heater hoses and turbo reservoir this way. Was chasing the problem for months after the engine rebuild. Car doesn't overheat, no bubbles in system, it just starts losing coolant between fillups. For my case, coolant would sneak out the heater hose clamps on the firewall. Which was where I disconnected them when pulling out the engine.