This makes me feel bad about my past behavior. I've walked in on past managers when they've been talking about my accomplishments or background with other managers or company owners. My instinct, and the way I was raised (military family), is to play it down and be humble ("eh, it was a team effort", or "I was in the right place and right time") or make some self-deprecating joke, but now I can see I was likely (unintentionally) undermining my boss.
I had already realized this wasn't a good idea the last time it happened, and resolved next time to just smile and say thank you or something similarly neutral. However, this comment underlines how bad of an idea it is to play down your successes at work, even if it's in your nature (I've also had people openly tell me to be careful about this re: my own career).
I just can't stand people boasting about bullshit at work, and want to avoid coming across like that. But I'll be more careful about playing down my own successes now.