There's not going to be any data, border agencies are as non-transparent as you can get.

Yes, or as we used to call it back in the day "empirically".

Plus, it doesn't matter if it happens as much or more in some backwater third world country.

It's already enough of a problem that among first world countries the US is quite a pain in the ass airport control.

Heck, it's enough of a problem that it feels like a pain in the ass. It wont be less of a problem if per capita e.g. Belgium or Albania are worse...