The Troll comes from the NORDTEST handbook on Internal Quality Control,  TR569, available here.

 
 Drawing by Petter Wang, Norway

Drawing by Petter Wang, Norway

 

Once upon a time there were a lot of Trolls in the Nordic countries like the one you see on the title page. They were sometimes pestering us so we have a saying something like The Troll is up to mischief meaning there are some odd things going on which we do not understand like a control value in the red area in a QC chart.