Over the years I found that smoke no matter how little will stain..The oily vapours mixed with flyash sticks to almost anything it touches. The sticking is more pronounced in a cold ambient.  It is a very slight amount that over time builds up.

+1. Also, everytime that you enter your deck from the house (during a cook) will let in some cooking odors. Pellet grills produce a light smoke so I routinely use smoke tubes for added smoke flavor. Have fun with your new grill!!