I am not averse to any particular technology (subject to cost, etc.). But, don't want to "alter" the environment, noticeably, by introducing obtrusive sensors, unexpected forms of radiation (e.g., visible light beyond what may -- or may not -- already be present), etc.
E.g., I could station a *person* as "observer" and get a reasonably reliable "detector"! But, impractical, obtrusive, expensive, etc. :> (not to mention allowances for "bathroom breaks"!)
Yes. Regardless of which "physical phenomenon" on which you rely, there's always a "gotcha".
I think I just have to prototype a few approaches and *measure* their performances -- get a feel for *exactly* what sorts of capabilities (and limitations) each has. Some other pattern may emerge from the data that leads to a better approach/solution.