"Even though we won't separate voluntarily, this is a precaution. The red string won't break and won't grow shorter. As long as the red string remains unbroken, it means the person on the other end is all right. It will surely lead us to the other, unless one of us is no more."

This is the road trip / beach episode before something serious goes down in the next. The calm before the storm,
AND THEY ARE BLATANTLY FLIRTING IN BROAD DAYLIGHT!

... they now had identical tiny red butterfly knots on their fingers.
"Now we're joined together."

You see my point???