Like the badges for script triggers, quickfind, tooltips, hide, etc., a badge on a layout, field, portal, etc. that would indicate that for SOME privilege set, there is a constraint set in the security model on that object.
It does not have to be any more specific than that; just a flagged indication, selectable in the the VIEW > SHOW menu, of a security model constraint against ANY privilege set; a pointer for an area to look at in debugging issues.
The security model is an awesome tool; granular and powerful - but also obscure. So many times, my "current self" is upset about my "past self" having set up some security condition, and forgot about it, ending up wasting a bunch of debug time looking for the behavior problem, when IF, IF, IF I had remembered (or had some indication of same) that a security constraint was set on the object, I would have known to look there as part of the debug process.
As it is, I avoid using the extraordinary power of the security model, as too often, it costs "future self" a lot of wasted debug time. I'd use it a LOT more, if the impacts were a lot more obvious.
Just a simple badge, of the same nature as other developer aids in layout mode, would be welcome.