At 6 minutes in, when Morse and Lewis drive at nighttime to Bay 5B, the rear view mirror of Morse's Jaguar has been removed. The mirror reappears and disappears on several subsequent occasions - but it is most evident in the first scene mentioned because Morse makes a studied attempt to look in this absent mirror near the end of the scene.
The trivia item below may give away important plot points.
The phone call which Rosemary receives at the beginning of the episode, in which she is told she is a 'selfless, thankless b****h', seems to be taken from a letter (with almost identical text) received by Margaret Bowman in Colin Dexter's The Secret of Annexe 3. This is itself a story concerning an adulterous wife, a blackmailing lover and a dead husband (the 'Secret of' tag is perhaps also more than just a coincidence)