Sounds like it was probably your MIL. I would keep some distance from her and avoid her at all costs, if for no other reason, the rude/sexist comments she made while visiting. Keep in mind that your kids are safe no matter what crap anyone makes up. You know the truth and it will show.Let it lay....
As long as you have nothing to hide, and there is nothing going wrong, child services isn't going to do anything. They have to check things out when a claim is made- but more often than not the claims are false- just business as usual to them.
We were investigated by CPS when my son was 3 months old because he had fallen off our (very tall) bed and fractured his skull. They came though, and found nothing wrong. They said it was state law that they investigate bone breaks and other injuries on children under age 2...and they were very concerned because he also showed an older break on his shoulder (his shoulder broke when I was delivering him) and the hospital failed to mention to them that the shoulder broke under their care so they assumed both had happened in my care.
It was invasive, and scary, but nothing came of it. They saw our house was normal and our baby happy and healthy and went on their way.
Also, you don't HAVE to let them in. They will pressure you, but you are well within your rights to deny them access. When CPS notified me that we were being investigated, I told them I would only let them in with a warrant. They came by a couple times, I denied them entry, and then they finally got a warrant and an officer came over with them. At that point I allowed them in to see my house and my child.
If you're really determined to find out who it was, you could try calling your MIL and telling her that Social Services released the name of the person, because the claims made against you were clearly false. Then see how she reacts. If she asks who, say "All that's important, is that I know, and I don't want to talk about it.". Tell her you will be talking to an attorney. That should give you an answer if it's her because she will get defensive if it was.
Legally, I wish there was a law that would force the release of the name of the person if Social Services found the claim to be untrue and malicious. Especially, if it's done more than once by the same person. It seems like you should be able to file charges against them for defamation of character, emotional distress, and harassment. Also, you think Social Services would be upset with people who waste time and money by making up crap just to be malicious (not people who's claims are legitimate or out of reasonable concern). There really should be something in place to stop these people who abuse the system. You have a right to be upset, and if you wanted you could call and talk to a lawyer and find out if you have any rights in the state you live in considering you were falsely accused. Hope you get to the bottom of it. Good luck