Just to chip in from the mil side on Balix's comment.
but we cannot be expected to spot every 'unknown' military aircraft.
Be you Mil or Civ it is dangerous control practice not to use radar if you have it and rely on the assumption that just because you are in CAS you know whats there. All controllers, irrespective of airspace classification, should not assume that the way ahead is clear. If you are not spotting 'unknown' traffic on YOUR radar in YOUR airspace then you are failing as a controller. Check and never assume.
Going back to earlier comments about flight strips. Mil guys are taught from day one that their main control aid is Primary Radar with Secondary thrown in as a bonus. We use flight strips as an aide memoir to back up the big picture we are looking at on radar. Unlike civil controllers, procedural control is only employed as a last resort when we cannot provide a radar service. If we have a radar we use it, and do our damndest to keep the little/big/slow/fast sods safely apart.