Have you ever spent time on the Border? While stationed in Imperial Beach California I used to sit on the hillside that faces the border with my uncle and we would watch as they would litteraly try to cross in herds. About every 15 minutes a border patrol officer would come through and check on them periodically to mark their position. Knowing that I was in the Navy, On one instance one of the officers handed us his night vision binocs. and asked us to keep an eye on them while he run up the pathway to check on another herd of them coming over. He returned about 15 minutes later with several other officers who waited them out until they finally snagged them trying to make their move.
I happens, and if you dont believe it, go have a look for yourself.
I believe it happens, and by the way, I live in San Diego! I was just saying that 1500 a day sounds like an awful lot.