People I have seen claims submitted for/enrolled incorrectly on the plan:
The next door neighbor (yeah, we found this when the neighbor tried to get pre-certified for a transplant)
Grandchildren
Children well over the age of being a student (25+)
The ex-husband and his current wife

The ex-spouse's children
Foster Children
A domestic parter
and a spouse (don't wanna know what was going on there)
And this one is the best....
The family dog
Seriously, I kid you not. A very famous person did this by the way. Someone who could
well afford to pay for the dog's bills. (Which totaled over $10,000 for when the dog attend a private "retreat" with the member

) Nope can't reveal names that would violate HIPAA. All I can say is the man had millions and millions in the bank.
So the fraud happens everywhere, every day. And it's costing insurance companies and especially self-funded plans a fortune.
ETA - allowing this fraud increases everyone's insurance premiums so if you know it's happening you should report it. All insurance companies have a fraud division where you can call to report things that should be investigated.
If your insurance company is conducting this type of audit, I would just confirm that the company performing the audit is legitimate, but you should know that these types of audits have been going on for years at self funded plans, just without getting the corporations and all employees involved (only the one's about whom we had questions - like the employee with a domestic partner and spouse).