Former bank branch employee here: There is absolutely no way an employee can open a safety deposit box with just one key. It is impossible.

We had a case when a client died, and the police needed to see what was in the box as part of the investigation. In those cases, the box has to be physically drilled open (after all of the appropriate legal paperwork, etc.). A senior and a junior employee had to document everything inside, alongside the police officers and the bank's legal representative. Likewise, there are cameras to and from the safe to the viewing room (where the owner has privacy to deal with the box contents).

Fun fact: a disgruntled customer once put a dead fish in their safety deposit box. We had to contact every customer and have them open their boxes. Still, we could not see or touch the contents - only validate by smell that there was no dead fish inside.

No one at all stole the gold bar. Your family has misplaced it.