I say that maybe the cop realized that weed isn't such a big thing. He just asked to do a search to keep it by the books.

The first time I was pulled over with some bud on me I was on my motorcycle. I think they were more worried about harassing me because I was on a rocket then anything else. (it gets pretty bad around here even though I'm one of the good ones) I got a 30 dollar ticket because my license plate was a few degrees away from being straight up and down. I had my stuff socked and there really wasn't anywhere to search other then under my passenger seat, so I don't think they would have even asked to search anyways. Although, I was coming down from smoking about 2 hours prior. I really wasn't nervous at all. I pretty much knew they couldn't do anything unless I admitted to it if they had asked.

Mostly it's about knowing your rights. It also comes down to whether the cop wants to be an ass or understands the situation.