I would think it's a joke but once literally had an office clerk in Germany scratching with fingernail my signature to check whether it's by pen and in the right color.
For more information, you can check whether the ink complies with DIN ISO 12757-2 and/or read up on "Dokumentenechtheit" [1]
[1] https://de.wikipedia.org/wiki/Dokumentenechtheit (It hasn't been translated to other languages yet.)
Certainly an interesting man. I highly recommend checking some of his work (ie. The Metamorphosis).