From the rules:
(you can Opp fire at) a hex in which an enemy unit expends at least one Movement Point (MP) by any kind of movement other than Low Crawl
or Stealth Movement
and
A unit(s) expending MPs (either entering a new hex, pivoting within a hex or unloading/loading passengers (16.0)) can be subjected to OF attacks equaling the MP-cost of the movement
Note that, even though a successull smoke placement attempt will reduce your movement allowance by one if you move afterward, it does not, in itself, constitute a kind of movment (until you actually leave the hex you were at the beginning)