The 135i's and every other N55 (or N54) motor has an ELECTRIC WATER PUMP. With an electric water pump the ecu controls the water flow. BMW opted for no coolant gauge bc the gauge needle would move around too much - for the average driver. And so not to confuse the average driver, BMW decided not to provide an coolant gauge.
We all just wait until we see the oil temp gauge start to come up to temp, before we start to hammer it!
Btw... the only other car I have seen without a coolant gauge is VW new Beetle. There you get NOTHING! Just a blue light when you first start the engine. AFter the engine is warm that blue light goes out.