When Dean saw Cas on the side of the road his first instinct was to pull over. He did, and Cas wasn't there. Dean was only a little disappointed about it not being real, but only because it happened so often. He saw Cas everywhere, in everything. He had stopped having real hope that his beautiful angel was no longer in danger and back by his side. He thinks Sam has noticed. Even if he brings it up Dean won't admit to anything. Sam will assume things. He'll make the mental connection that he too saw someone everywhere he went, and that person was his one true love. He'll be right of course, but Dean won't tell him that. Dean won't tell Sam anything. He'll just wait till Cas is actually back, because then everything will be okay.