UPDATE :

Today I got up above the drop ceiling, using 2 ladders & prop rod.
When the shop was built, they made some 3" holes next to every truss to vent into the ridge cap.
While the cap/roof gaps are maybe an inch tops, apparently that's been enough.
I could see a trail of peanut shells & insulation running down from those holes to the drop ceiling.
So, I stapled some wire mesh over each hole.

Seems almost silly now, but man, squirrels sure can squeeze into tight spaces !