When Mathias first visits the murder scene, which is in daylight, he finds meat in a freezer, which is working, and he goes into the darkroom and turns the light on. So electrical power is clearly connected, even to this remote farmhouse 'on the mountain'. When he returns by night to check things, he looks at everything by torchlight, and does not turn the lights on.
At 00:26 Downey Has An Up- Ended Refrigerator On His ATV For Mathias. The Fact Is: If You Store A Refrigerator (Or A/C Unit) In Such A Fashion You Will RUIN It.
DCI Mathias walks into the victim's house already wearing the protective blue booties issued to the UK police. They're dirty from mud, which would contaminate forensic evidence.
The booties are always put over footwear at the entrance to a crime scene.
When DCI Mathias resets the vinyl record at the crime scene, the subtitles say "(Italian opera plays)". In reality, Franz Schubert's song "An die Musik" (Op. 88, No. 4) is playing.
While the box of children's teeth found in the care home is treated as ominous and evidence of abuse none of the teeth have any roots, meaning they were removed only when the discomfort would be the minimum possible.
When the villain races away from his house in an old van, the police chase him 100 feet or so on foot before giving up the chase. The foot chase takes them right past their police issue turbocharged Volvo which could have easily kept up with the escaping van.
In the English version, when Mared Rhys is sealing the envelope at the end, one shot clearly shows the envelope addressed in Welsh, but in the next shot it is in English.