[vmception]

What kind of source would you be looking for

Its like cellular service in an underground subway station: people tell you it arrived and use it and nobody thinks about it afterwards. There wont be a source about that whether you believed it or not.

USDT is as frictionless so it just has use.

[RC_ITR]

I mean, all functioning governments track the net value of imports / exports (that's the main job of customs departments).

Most functioning governments also keep a balance of payments (https://www.imf.org/external/np/sta/bop/bop.htm).

If the flow of imports / exports doesn't line up with the net flows of payments, someone would raise a flag.

So yeah, unless it's ML / illegal trades, we could at least figure out a way to track it.