I think it is a widely-held opinion that currently, fortifications such as bunkers, trenches and foxholes can be a liability as much as an asset, since they are often spotted well before the troops that are supposed to be hiding in them are, leading to "speculative" area fire that renders the defensive position uninhabitable. My hypothesis is that fortifications get no benefit from their "experience" level for concealment (since experience soft factors don't affect the price of the fortification). It's like how you can often see a dead member of an enemy team but not the live ones, even if the casualty is prone and therefore harder to see. At least that's my hypothesis.
So, my suggestion is to make the Experience level (and maybe the leadership level) of a fortification affect how hard they are to spot in the same way as those soft factors do for infantry. This would represent a better-built fortification, with better blending into the local flora and topology, and at the very least would give the "benefit" of "Conscript" level training, which is better than "none" (as no-soft-factors-applying entities like casualties currently receive). It must be technically possible to allow only a limited number of soft factors to be considered, since artillery doesn't consider Morale or Fitness when calculating call times and accuracy (and QB points).