I don't see a contractor equipped to fix some lights make their way out with a few hundred pounds of lead, on top of that they would endanger their career and reputation for some 10's of $ whereas the job they were there for probably pays a large multiple of that.

Blaming the contractor is easy, but the vast majority of such thefts is done by insiders or security personnel.

This is exactly what was thought to have happened to the previous low background lead supply. It sat around in a big empty lab long enough for someone to notice it and notice that it was in a mostly empty lab.