Unfortunately, that's not an isolated case. It happens fairly regularly, and most of them go unreported.
Actually, that should be for any IT staff, not just senior, and doubly so for anybody with any level of administrative responsibility. Not only do you disable their accounts, you also make sure to terminate any current open connections. Yes, that's happened. A senior IT staff was terminated, and all of his accounts were terminated. He still had an open VPN connection to his home, and that gave him enough access to cause trouble.
oh it happens frequently I'm sure. THe only differece here was that it happened in the public sector where they can't sweep it under the rug.