It means "put your hand into pocket and grab something "

一......发现.......,means when you do someting,only to find.......(express that you are kind of surprised)so in that sentence,it leaves out"发现". i think the verb"掏" is equal to"scoop" .if anyone knows more proper translation,please share.

就是你把手伸进你的口袋里，然后把口袋翻出来了，就是一掏兜。是个非正式用于吧。