I've seen people give rated seeks, but their rating doesn't show. I've had people without ratings answer my rated seeks. It is very uncommon, but I've seen it. I don't know how they hide their ratings.
After playing one of those people, I fingered them and found the rating. Perhaps they were new and were provisional, I don't know.
If you don't want to know your opponent's rating, the play with Winboard. Just don't look at the text screen until after you've finished the game.