What would you do if an ex-coworker lied, and told your significant other that they slept with you?

I am in a bit of a dilemma that I am not sure how to handle. I worked with this guy for a few years, we will call him Bob. Bob and I never worked closely together, there was never any drama and we would chat politely from time to time about nothing of significance. There was a time where he messaged me of face book and I responded politely but remained kind of distant because I didn't want to encourage or give the impression I wanted anything further than to be colleagues.

A month or two ago he left, I am not sure if he quit or was let go. I never asked. Last night at about 4 in the morning my husband got a facebook message from Bob, where he claimed to have slept with me while he was employed here. His accusation is entirely false and I have no idea why he would make such an accusation, or what is going on. I tried to look up his facebook and bring up the old messages but it would appear that he has blocked me on facebook.

My husband is suspicious, rightfully so, I would be too if I were in his shoes. He messaged Bob back asking for proof, which I encouraged him to because there isn't any.. I am angry, hurt, and confused because I don't know why Bob would do something like this. I don't know if I somehow offended him, or gave him the wrong impression, or if he was drunk at 4 am in the middle of the week...

I could reach out, he has me blocked on social media, but I work in the business office at my job and I could get his information. The angry part of me wants to track him down and confront him, but he is already acting crazy and I don't want to make things worse because who knows what he might do. What would you do in a situation like this? How do you think I should deal with it? I don't know what to do...
What would you do if an ex-coworker lied, and told your significant other that they slept with you?
Updates
+1 y
Thank you for all of the advice. After careful consideration I got his phone number from one of my coworkers and I called him last night with my husband present, and calmly asked if his Facebook got hacked because I got a weird message...… Turns out he had just broken up with his girlfriend and his ex decided to be a complete psycho. All is well now!
What would you do if an ex-coworker lied, and told your significant other that they slept with you?
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