I'm not trying to be a smart-@$$; however, Do your in-laws have CID by any chance? If so, maybe they were just too upset to look back through it and see what number called them.
Definitely, do what SOAPLADY suggests, if your in-laws don't have CID. If you still can't figure it out then be patient, if they called your in-laws once, they'll do it again.
You may want to educate your in-laws that what this CA has done has committed their first violation of the FDCPA by telling them your personal business like that. Also, the threat to come to your job and "humiliate" you in front of your co-workers was a big violation too. Tell your in-laws to tell the CA not to call them any more. That SHOULD be enough to make them stop; however, it probably will not. If they call them again after they are told not to, then your in-laws may need to put it in writing. Or, once you find out what CA it is, you could put it in writing that they are not to contact any third parties regarding your business, ever.
Just a few suggestions. Keep us updated as to what happens with this, and if you have any other questions, don't hesitate to ask.