| All that is when a voltage of 30kV and above was present and had the chance to accelerate higher amounts of electrons (= higher currents at the same time). With the lowering voltage the "efficiency" becomes so low, it is not any hazard anymore. Plus with the lower voltage than the ~30kV limit the energy of the generated photons becomes so low, it can not penetrate anything. So yes, a meter could respond to them when placed just close to the emitting place, but e.g. the regular wooden cabinet already reduces it so, you need long time to detect it...
For the tubes designed to generate the Xray on purpose, you have to use hard metals (tungsten,...) for the anode and make sure they do not warm up during operation (rotating anode,...), to get reasonable output from still reasonable voltage (50..100kV for the medical use)
In the TV's, the warnings and shields are there really to just cover the failures and maybe warmups, when the dangerous voltages (from the perspective of their potential to generate dangerous levels of X-ray) really could be present.
And the plastic covers are there for another reason: The CRT is a big vessel with vacuum inside. So when the glass crack, there is quite high energy "available" to throw heavy glass pieces arround...
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