Or a ladder. Nobody stops a pair of people dressed like laborers carrying a ladder.
They spooled up a truckload of copper, gathered their cones, and drove off. They were seen by hundreds of witnesses in the middle of the day, but nobody suspected anything.
You want invisible? Be female, 40+, short but otherwise average build, any skin colour but white, black hair, generic cleaning staff outfit, and one of those carts with mops and buckets. Now you're invisible (racist, classist, and misogynistic biases exist, might as well use them).
(As a 2m tall person I can't blend in anywhere.)
I didn't ask whether they actually worked for that company, or the outfit was part of their pen-testing toolkit.