In Sweden, if you accumulate points while travelling on business but use them for personal flights, the tax department wants you to pay tax on the value of the benefit (likely: how much you would have paid without points). How they can know about it is another matter.

I think at least the larger employers will ask that you keep separate work and private frequent flyer accounts for that purpose, and use your work points only for work flights.

Oh how I miss the Swedish tax system...