What sort of aircraft maintenance operation has such lax security in place so as to allow someone to wander around unescorted and take a screwdriver from a toolbox
That comment, if I may say so, is unreal. This is maintenance of general aviation aircraft, not maintenance of US$1BN (or whatever the price tag is) super secret super stealthy B2 bombers.
Passers-by nicking stuff (tools, headsets, even avionics) out of open hangars, and from parked aircraft, is a relatively common occurrence, just as stealing laptops and other valuables out of checked baggage is absolutely routine everyday occurence at major (and supposedly very secure) international airports.
The only way to prevent this happening would be to have military-style security around everything. Not possible.