A young brunette drew a smiley face on steamed glass. After the brunette is shown dead, the smiley face is shown again with an angular drip running through some of it. It, suspiciously, resembles the smiley button with a blood streak from "The Watchman".
Nicholas Gleaves (Tim Hopkins) and Richard Riddell (Robert Doran) also worked together on Bodyguard (2018) as Roger Penhaligon and Tom Fenton respectively.
Nicholas Gleaves (Tim Hopkins) and John Light (Andrew Franks) have both appeared in the series Father Brown (2013).
Nicholas Gleaves (Tim Hopkins) and Richard Riddell (Robert Doran) were also series regulars in different seasons of Scott & Bailey (2011), with Gleaves playing DS Andy Roper in seasons 1 and 2, and Riddell playing Steve Dench in season 5.