I wouldn't read too much into it, as big as the radiator is and as small as the expansion tank in relation is, the level is going to bounce around just from a few degrees change in temp.
I did have a little leak around the water pump when mine was new but it was so easy to find because of the obvious crust visible on the bottom of the pump.
Be sure to tell the shop when you take it in though, your complaint gives them authorization to do a pressure test just to be safe.