Oh, combustion smoke maybe, but sodium rocket exhaust? We're talking damn near radar opaque.

And if your laser is passing enough energy to visibly ionize the atmosphere, then you've reached the point of diminishing returns: that same plasma is going to absorb a good portion of your beam.  

I'm not sure what you mean by serious laser either. I've seen those Israeli iron-dome units fire, and there's no visible beam. Despite the fact that these things slag freakin artillery shells. Plus there's that pulse ionization laser in Texas they use to create lightning paths...

Mainly though, rather than displaced air, I was thinking the kinda snap crackle pop you get from capacitors.