What does it mean when a guy flirts under his breath?

I’ve caught my co-worker who I have a little crush on say a few flirtatious things after I would say something. For example, one time he was dancing and mentioned that he isn’t that good because he has a pudgy stomach, and I told him that I have a pudge but I can’t dance at all. Then he mumbled something like “You have a nice natural pudge” and I asked him to repeat himself so I could make sure that I heard him clearly but he said nevermind. Then another time this had happened after I sat with him in the breakroom and I let out a sigh because I was hot and tired. He asked me if anything was wrong and I said “Nothing, I’m hot” and he said “What?” wanting me to repeat it (even though I said it loud and clear enough the first time but I repeated myself anyway). Then he mumbled “You are hot.” and assumed that I didn’t hear him. He is nice though and helps me out a lot at work even without me asking. I’ve also caught him staring at me a few times, not in a creepy way, but more like he hope to get my attention type of stare.
What does it mean when a guy flirts under his breath?
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