Probably depends on where the landing pad is located. The ocean based pads would have very little dust. Also if the rocket takes off from the same pad and lands back at the same one (like a lot of the tests) then the launch itself blew away most of the dust, so when it comes back, there's a lot less. The falcon heavy video that I linked to shows the return stages landing in a *different* location to where they took off from, so lots of dust.