The brief
Mr Raja's 520i was losing coolant and he couldn't see where it was going, no puddle under the car, no obvious weep, so he brought it in for us to find it. Coolant that vanishes is going somewhere, and a leak you can't see is the kind that catches you out, because the day it lets go you're stopped at the roadside with a hot engine. A common BMW coolant leak is the expansion tank, the plastic reservoir that holds the system's spare coolant and lets it expand and contract. The plastic sits under pressure and heat cycles for years until it gets brittle and develops a hairline crack, usually around a seam or the level sensor, that weeps under pressure and barely shows because the engine heat dries it off. A cracked tank doesn't reseal, so finding it takes a pressure test, and fixing it means a new tank.
The diagnosis
A pressure test on the cooling system held the system at pressure until the leak showed itself, the plastic expansion tank was weeping from a hairline crack, which is the disappearing coolant with nothing on the ground. The radiator, the hoses, the water pump and the rest of the system held fine. That's an expansion tank replacement with a fresh cap, rather than chasing a crack in brittle plastic that's only going to spread.
The work
The cooling system was drained down, the old cracked expansion tank removed and a new genuine BMW-spec tank fitted with a new cap and the level sensor transferred across. The system was refilled with the correct BMW coolant, bled the proper way so no air pockets were left, and pressure tested again to confirm it held with no weep. A road test confirmed the gauge sat steady, warmed up at the right rate, and the level stayed put.
The outcome
No more coolant loss, the level holding between checks, the gauge steady, the engine warming up at the right rate, and the system holding pressure. The 520i went home with the leak resolved. A cracked expansion tank only splits further, and the failure at the end is a sudden coolant dump and an overheat, so the pressure test that found it and the new tank kept it to a tidy, planned job.