Well, I still have a hard time seeing them establishing a real proof if private and company paid points mix. But for sure they will try. I guess they will send a questionnaire:

We see you earned x points in 2017 and you spend y points in 2017. How many of those earned points where earned on business and how many of those points spend were spent private? You have 2 weeks to answer.

And then they will come back with a tax decision based on some sky high numbers put on the value of one EuroBonus point. Which is the moment I'll my lawyers to build up a defense around the actual benefit value.

EDIT://

funny enough, SK has finally reworked their website it seems. Haven't seen it for a while

If you click on "Export to Excel" next to "Lastest Activity", it exports a quite nice list of your earnings and spendings since... tadadadada... 1/2017. Coincidence? Don't think so. Anyway, just looked through it and funny enough, I did spend no points on private travel in 2017. So should be good there. Anyway, I assume since they now targeted 2017, the following years 2018, 2019 and 2020 will be requested next.