Put on a high visibility waistcoat and carry a clipboard and nobody is likely to question it. They could, of course, have used a laser measure, but I've never found them as accurate as a tape measure.
First, they could, if they wished, take several nights to attack from underneath without leaving any visible signs. Second, if, as suggested, the machines dye mark the notes if the safe is forced, that might be an area which, until now at least, has not been included in the protection.
Colin Bignell