rust on top of the receiver?? unless ruger SEVERELY changed the design of the 10/22, that should not happen.....
Aluminum does NOT rust (does not turn to ironoxide). my guess (having not seen the rifle) would be that either, that was some form of grease on it (this is what i truly suspect) or that rifle was not as "new" as they lead you to beleive (even if they DID bring it out in a box, you can put one back in a box).....
if it was EXCTLY in the area where the scope mount rail would go, i'd suspect that someone put the rail on receiver and the gun got left that way in moist environs.
like i said on a 10/22 the receiver is an aluminum block, the color comes from the Anodizing process (same way a MagLite gets it's color, and surface hardness) so it should NEVER rust, and because of the anodizing be fairly resistant to oxidation of the aluminum as well.
something's fishy here, if it WASN'T grease i'd be getting ahold of someone in chage at that store kinda pronto.
BUT i am seriously betting on it being a orangish colored packing grease.. was there a small translucent orange "smear" on the inside of the plastic pouch the gun should have been in inside the Box??