Does a concealed/dummy unit force the opponent to reveal HIP fortifications when it has LOS to them (within 16 hexes if in concealment terrain) or does the controlling player need to reveal a real ground unit to do so?
I believe you would need to reveal a unit - dummies are not "Good Order" units:
A12.33 FORTIFICATIONS: Fortifications in Concealment Terrain may always set up hidden and remain hidden until use of its protective TEM is claimed, or a unit therein loses its concealment, or a Good Order enemy ground unit gains a LOS to its Location and is within 16 hexes.
Note, dummies are grouped among "unbroken units" (see A12.1).
When this conundrum presents itself I usually say to my opponent that I have Fortifications that may be revealed if he wants to temporarily reveal units (not necessarily saying which "?"-stacks he would need to reveal something from yet).
It become a little more intricate if the LOS is not a given.