"...[W]ere watching TV" means that the couple did so in the past and finished doing so, using "had" here would mean they probably only stopped watching TV to speak to the police officer, or some other connection to the present. Or so I take it; I'm still learning myself :-)
Here is further explanation by IMcRout on The Free Dictionary English Forum on a similar question:
As I have been directly addressed I shall not refrain from doing my
duty.
has / have been sent ---> present perfect tense
used when the action has an immediate connection to the present. It is
very often used with words like just, already, always, never, this
week / month / year ...
The little word 'just' in the first sentence indicates just that
connection.
was / were sent --> past tense
used for an action that began and ended in the past. It is often used
with ago, in 1975, yesterday, last week / month / year ...
The tense is often decided on by the speaker's attitude towards his
deed:
They were sent ten minutes ago. --> It's over and done with. Let's
tackle the next problem.