This Wired news article reports that
A five-nation tax enforcement cartel has been quietly cracking down on suspected internet tax cheats, using a sophisticated web crawling program to monitor transactions on auction sites, and track operators of online shops, poker and porn sites.
The countries participating in this Xenon project are Austria, Canada, Denmark, The Netherlands and United Kingdom. They are in co-operation with Amsterdam-based data mining company Sentient Machine Research.
A very interesting detail is that the search process is very “slow” to prevent finding it in server logs!