HD, you miss my point and you may be out of touch with current NATS safety concerns. Your original comment was...
What gets ATC concerned is infringement of the CTZ because that airspace is from ground level and anything "unknown" in that airspace causes major problems and they have to take avoiding action with their traffic.
Primary only traffic in a CTZ, it must be infringing therefore avoiding action will be taken (assuming it is seen, some aren't for varying reasons). But, it is known that it must be infringing because the airspace obviously starts at ground level ergo although of concern it is not as great a concern as...
Sorry if I did not make myself clear but Warped Factor knows that non-squawking traffic goes up and down through WOD quite often without ATC taking action as it is assumed to be below CAS.
That is the major concern! Primary only traffic, or traffic with no Mode C, that is as you say assumed to be below a base of CAS but in reality isn't. ATC will take traffic directly over the top of such returns in the assumption the traffic is below the base. Unfortunately it all too often isn't and there have been some very narrow escapes from worst case scenario which, of course, we've only found out about too late.
It's the one you don't know about that will get you, and that will almost certainly be a primary only, or no Mode C, vertical infringement taking out something much bigger inside CAS because no-one will know it is where it shouldn't be so no chance of electronic or human intervention to save the day. The best chance of avoiding that is to use Mode A and C or S at all times, mandated if necessary.
There have been enough close shaves of late that luck is surely running out.