helium leak detection is a difficult job.
It took the Rye House power plant 6 weeks to find the leak. (Loose flange of a pressure test line)
Helium leak detection is the best way to find leaks but sometimes it takes time to find it.
we have experienced leaks in bellows as well, especially with multi layer bellows.
Also the turbine exhaust area could give problems, such as near the gland steam or the casing.