The Sixty four dollar question?, I would think the answer as to holding water or not (visible in the bowl) would depend on:
1) The height of the intake hoses and out let hoses
2) Do they have vented loops in both lines
3) Are the vented loops free and clear and not blocked(when were they checked last?)
4) The general hose lay out/routing, correct angles,any loops etc,
5) Are the toilet non -return valves closing(these quite often get clogged with calcium build up which then allows water into the bowl)
In a normal system you will get a little water returning from the vented loop lines (all though there are non return valves to supposedly stop water from flowing back into the bowl) but water should still be present in the line to stop the gas/smell coming through the bowl ,
Cheers Steve