Found out a teammate hired six months after me is making about 15% more for the same title

Comp came up sideways in a conversation and I did the math after — someone on my team, same title, six months less tenure, making roughly 15% more than me. Not sure if it's a market-rate thing for when they were hired or if I just negotiated badly at the time. Don't really know what the move is here — ask HR, ask my manager, or just quietly start interviewing elsewhere and let a competing offer do the talking?