无处不在 follows the 无...不... double-negative pattern meaning 'there is no place where it is not present' = it's everywhere. Compare 无所不知 (to know everything) and 无所不能 (omnipotent).
memory
Break it down: 无 (no) + 处 (place) + 不 (not) + 在 (at/present). No place not present = present everywhere.
