Coolant stop once a week can't find leak

[Beanmk5]

Been getting a STOP coolant warming once a week and just been filling with water, obv with weather changing I want to get some antifreeze in but wanted to find leak and fix it
Ordered some of this stuff, u put it in coolant tank and run car then shine uv torch and it glows like a glow stick but can't see no drips or any sign of a leak
So now I'm baffled

Only thing I have noticed when it says stop coolant low is the screw on cap isn't tight any more so bought new cap and still have same issue

[GVK]

Some garages will have a coolant system pressure tester, to pressurise the system to show up leaks

[bobby_fodge]

Some garages will have a coolant system pressure tester, to pressurise the system to show up leaks

This test found a tiny crack in a coolant flange that was the cause if my mk4 losing coolant. The test only took them 20 mins.

[breeze]

Depending on how affordable your garage is, it may be worth buying a pressure testing kit on Amazon. They are not expensive, just a pump with a pressure gauge and a couple of adapters.