To be fair, quite a few people probably give you a blank look because they don't have a clue why you're waving at them. Just because they own a Scooby doesn't mean they have heard of the 'waving-at-other-scooby-drivers' thing. They might not be a member of the SIDC, or a member of this or any other forums, or involved in the Scooby community in any way.
The only time I can understand waving is when you know they're a member of SIDC or something, either because of a window sticker or you've met them before. Also, obviously, if they wave at you.