Totally anecdotal, but I was once pulled aside on the jetway by a woman with a Treasury badge and a burly guy who was probably a sheriff though he never said anything. She wanted to know how much cash I was carrying. I was flying from SF to Frankfurt, and I think it was 1994. My net worth then was about $30k, so I laughed and told her I had about $200 and a credit card.

So maybe CBP doesn't check outbound travelers, but Treasury did at least once.