In both those cases, there should be no problem with letting the person's attorney know what this is about. If it's a clearance investigation, it's a no brainer. The government has no problem leaving voice mail in Peter's mailbox saying "we're trying to reach Paul because Bob's applied for a security clearance." So this kind of secretiveness is unnecessary.

If it's an ongoing investigation, having the lawyer there is a GOOD THING, because it becomes the lawyer's job to keep his client aware of the repercussions of doing anything to sabotage the investigation.