Pressure building up in the radiator has the coolant hose outlet by the rad cap to enable overfilled rad coolant to escape.
Maybe it allows some coolant at high revs to escape or maybe the rad cap pressure spring maybe worn.
There is usually another rain water outlet tube coming from the petrol cap position.
Both are routed to the rear of the bike.
A lot of this stuff for Thais who can speak English or not speak English will find it hard to explain, it sounds like there taking care of you pal.