Your boss has loose lips

Privacy is an illusion.

Your boss must have shared it with someone. There is no other explanation.

Your boss likes gossiping..

there are NO secrets...none...never ever

According to my deceased grandmother "a dog that'll bring a bone will also carry one!!" Meaning if you don't want it known then don't tell it to anyone not even your supposed good friends because you don't know and have no control over who they tell!! If you're at work then you really need to keep your mouth shut because many people are just looking to get in your business and report it to others for personal gain so they can get ahead!

U already answered ur own Question. Don't take a Genius to figure it out.