Defence in Depth

The criteria is that the first kind of line is staffed by
people with a radio, to tell the folks managing the whole
mess that they've encountered the enemy, and in what strength.

In our case, one might do a logical variant: provide a master
password mechanism, and use "not unlocked" as a warning to other
security mechanisms that the owner thinks they're NOT doing
something insecure.

That will definitely catch "probing" attacks, just like a "tripwire"
defense line does.

--dave