Here is me making assumptions. Oops! When you said 4/5 gas detector I thought you meant one of those meters you use for confined spaces. Where detecting HC was done by using the LEL function.
My understanding of these meters is all classroom based as the company policy is no crew are allowed to enter a confined space. Shortfalls of doing your cadetship and first 3rds posting with the same company.
From your last post I understand where I went wrong. My father is domestic heating engineer and uses these to test oil boilers. I suspect he won't mind me borrowing said tool for a trip
Interesting point about using the tester to measure smokers breath, My chief smokes more than the engines do. At least 10 with his morning coffee. So it would be amusing to test the smoke level in the ECR at this time.
Your point on 'heatup of water, seals the offending part' is our theory too but we have had all possible offending parts pressure tested and there is no evidence of blowby between gaskets.
We have found why the water level is rising, air is getting trapped in the top hose which is above the venting hose to header tank. If we manually vent the top hose when she is warming up, no bubbles or raising level in header tank. As to how the air is getting into the system is still a mystery.
If we cannot get a smokometer on board in next port of call I suspect the best next step is to send a water sample from each Aux engine ashore to be analysed in lab to help narrow down the search. This would also prove if the dirty water is rust or HC.