Pee leak mystery

Hi mummies, would like to seek help to solve my baby boy pee leak mystery. I’m a first time mum and my baby boy is turning 3mths. Recently we would find his left side of his back (above the diaper) wet. Suspecting is pee leak, but don’t quite understand how it leaks coz the diaper seem to be dry on the waist line and the diaper is only 20-30% wet, so definitely not leaking out due to diaper full. He is wearing MamyPoko tape diaper size S which is appropriate for his weight. So appreciate if any mummies who had similar experience to help us understand how did the leak happen 😅