It's not a criminal matter. The employer might bring it up if there is an employment dispute. An employee who regularly comes back from vacation injured or sick is likely to raise some eyebrows (similarly for coming to work every Monday with a massive hangover), but it's totally at the discretion of the employer if they want to do anything about it.

On the other hand, we do get separate sick days (technically even if we get sick during vacation, but I don't know if anyone actually tries to get vacation days back they lost due to sickness). It's very different from countries when you just get time-off days and have to use them for both holidaying and covering sick leaves.