It's not uncommon at all. Although in Korea, people are pretty reserved in public, in certain settings it flirting happens on a regular basis. You can already guess what those are - clubs, bars, etc. Especially in clubs you will see guys approach girls constantly. My girlfriend took me to club NB 2 in 홍대 and I saw tons of it, partly because the guys are liquored up, and partly because it's accepted behavior there. I wouldn't recommend trying it anywhere else though, or you might come off as a creep lol.