Switching and non-switching period in buck converter
Looking at the data sheet the only reference i can find to low power mode is pulse skipping. The switching frequency range is quite wide and all quite high so if you have very little load at the output it could be skipping a lot of switching cycles when in it's 'Eco' mode.

If you think about the logic of the situation, if the datasheet says it doesn't have any other way of providing the output voltage (say an internal LDO which is more efficiency than switching at very light loads) then the the voltage at the output can only be maintained by rail capacitance and/or the regulator switching intermittently.

What switching frequency have you set it to and what value inductor are you using?