In All About My Romance, episode 11, An Hee Sun(played by Han Chae Ah), who liked Kim Soo Young(played by Shin Ha Gyun), witnessed him and Noh Min Young(played by Lee Min Jung) kissing each other.
Hee Sun followed Soo Young to the seminar at a province to check on him and Min Young. She has been noticing a romance between them and was anxious that she might lose Soo Young. She has also warned Min Young previously not to interrupt them.
However, Hee Sun eventually found out that they were dating as she witnessed them kissing each other at a mountain.
Source : Koreandrama