Confused. What clued them in at the end?

Also wanna know what the fuck happened on those last panels lol what is this why is it so damn slow to progress dammit im confused

I think it’s the text message. She replies Yes. And it doesn’t sound like her. Last chapter Boem was thinking about how cutesy her messages are.

Taeha gave his guard clear orders. Look around, enjoy for yourself, return to haesoo. That's it. He didn't give his guard any further orders. So when he his bodyguard he knew that somebody is about to mess with his girl. Since bodyguards work on these lines their instincts are sharp. The guard now knows someone tipped him wrong info so that he would be late

Because Beom told Jay to hang around the party for a bit and then check on Haesu.The other guard was trying to distract Jay and lied about Beom wanting to see him. When Beom sees Jay and essentially gives him a “wtf are you still doing here” face, they both realize some fuckery is going on.