Dean has so many pockets, he could have added one in just for the umbrellas. And since Sam doesn’t have that ever-useful knack of finding things like a smol Dean would, he’s forced to deal with regular senses to try and track them down.
Naturally, he finds nothing, and the next time it rains he gets one of the flowery pink ones. He only uses it because he’s in Dean’s pocket and now it looks like Dean has an umbrella in his pocket instead of any pens.
Brothers Apart 