Answer: Confidentiality / Non-Disclosure agreements
Step-by-step explanation:
We've all watched movies where people discuss very secretive things over the phone or with another person while they are being driven and at some point we wonder why those discussing are not worried about the driver spilling the beans.
The reason is because these high-profile clients need to protect their privacy and so make sure that those working for them especially their security agents, sign Non-Disclosure or Confidentiality agreements that require them not to speak of the things they hear or see about their clients.
Should they breach this agreement, they will be in a wealth of legal problems as well as suffer reputational loss. Sometimes though this does not seem to work and they still breach the agreement. It is a good deterrent either way.