Simple, place a neon indicator or 120v rated led in parallel with the
switch, with router plugged in circuit is complete to lamp (only small
current need to light neon or led) with unplugged no circuit.
"Shut up and keep diggen"
It took a while, but we got some clever solutions.
However, it was pointed out that this is subject to false negatives, which
are dangerous. How about the opposite; it turns a light out when the router
is plugged in. That gives only false positives, which are harmless. You
could even use the light to help you change the bit!
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