CBP has an alert on my passport and I'm flagged as being a bad-boy with insane amount of grilling (and illegal jailing, strip-search, ~16 hour detainment, the works) every time I enter the country. Even I have never been searched leaving the country. I'm sure it happens but I think it is quite rare. I repeat myself, there is effectively no customs leaving the country even if you are flagged as most-high-risk as I am.

>CBP regularly uses cash sniffing dogs for this purpose.

The dogs are just probable cause generating machines. Had one of these dogs "sniff my asshole" and the agents got a warrant for me to forcibly be taken to the hospital to be "internally examined." (and I was sent the bill by the hospital, both ERs they took me to ~60 miles apart even though I never consented to any of it) The dog never alerted. It's just cover so they can get a warrant whenever they want.

Yeah I'm aware it's illegal to not report on the way out, I'm only saying it seems unlikely criminals doing that are going to get caught.