Mercedes gives a little hint that is quite hard to understand unless you’re familiar with the printed version of the comic, which so far only exists in German language.

So, here’s the point: Mercedes doesn’t have the best of relationships with her mother who, after her husband had died, never much cared about his grave. Mercedes loved her father very much, which we learn about in the first part of D&M and she still feels guilty for not having answered his last phonecall before he passed away.