Yes, I have sniffed header tanks before, it's certainly not a weekly or even monthly happening but provided the smokemeter is calibrated correctly and it's not a Mickey Mouse one ( looking on the net I see loads of them out there ) you should be able to detect any HC's present in the coolant system.
Provided you keep the smoke head detector clear of the water level ( as it draws a vacuum to suck up its sample ) you should be ok to measure any unburnt fuels present in the coolant.
Where I am at we have two smokemeters which are certified on an every year basis and both have been updated with the latest software as some of the newer engines are hard to measure the smoke output levels.
Their primary use is of course smoke emissions but for example you can measure a smokers reading by his breath output ( just thought I would drop that into the conversation )
The smokers reading will clearly come up on the screen which you can then print.
Haven't yet worked out how to tell which make of cigarette gives what reading so you can tell if the sample supplier is a Woodbine fan or not, never played with that idea as yet.
As your header tank( s) bubble and throw out water on startup I would consider that maybe the sealed coolant system is, to some degree being " pumped up "
As it doesn't continue to do this after running up the offending part might, on heatup of the water expand to seal the offending part(s)
Don't know about the discolouring defect but once again it's being introduced into the system from somewhere.
Again I see it as a diagnostic process eliminating all possibilities until you arrive at the correct answer.
After all the expansion in the header tank is being created from somewhere to bring about the venting.
And bubbles do not come from no-where, they are again being created.
Why it's both engines seems weird, that's why I asked if the same person performed the refit.
Also a pressure test will indicate why the cap lifts to vent ( don't forget the pressure tester can be used to test the cap and see at what psi it lifts.