RIHANNA'S 10-bus caravan was stopped at a border crossing near Detroit yesterday, after an investigator caught a whiff of marijuana.

A drug-sniffing dog was brought in, and somewhat surprisingly, they only found a "small amount" of weed on ONE guy.  Rihanna wasn't on any of the buses. 

So, there are 10 buses full of Rihanna's touring entourage, and there's just one guy with a little bit of pot?  Is that dog defective, or just low on batteries?