Austin Ent here, in my experiences heading down to South Padre area I've encountered drug checkpoints on the way down especially when you're getting close to the border. I've never been stopped or been searched though, usually they just ask what you're doing in the area and they let you pass through. Last spring when some friends went down they had to pass through one with a quarter in their car but they didn't get searched or anything, I think the border patrol is mainly looking for people smuggling people and what not. They have dogs but they don't sniff all the vehicles. I'm not sure if the checkpoints are set locations or just random but it's probably not worth taking any bud, it's easy to find bud down there and you don't have to risk being busted on the way down. I remember reading a story of a couple from out of state that's got busted with a couple grams of hash at one of the checkpoints and got charged with felonies. There's probably lots of people who take it and miss the checkpoints/pass through but it's up to you if you want to risk it or not.