LADIES AND GENTLEMEN, WE FINALLY HAVE AN ANSWER!!! thank you, guys!
When Sam was at Stanford, everytime a case took Dean relatively close, he would go check up on him. He would drive to the school, sneak into the dorms when Sam was in class and salt the doors. He would also leave any money he had somewhere in Sam's room, usually in the hollow book Dean got him for his birthday when they were kids. Sometimes he would even sneak into a massive lecture hall just to see his dorky kid brother's face.