The definition "when we want to say that something happened before some other event in the past" should not be taken as implying that an event in the past has to be mentioned in the text by means of a verbal form in the past. A Comprehensive Grammar of the English Language defines the past perfect as the tense that has
"the meaning of 'past-in-the-past', and can be regarded as an anterior version either of the present perfective or of the simple past".
- (user LPH's italics) Their reliance on him had been the unwritten contract between men and women in places like Wilshire. Husband works. Wife tends to the house, children, and the husband's needs. And she had done that, produced three chidren, overseen their care, managed the house. She had cultivated one of the most envied social life in Wiltshire. (Social Lives: A Novel - Wendy Walker · 2009)
T3 (before then); T2 (then) T1 (now)
(CoGEL) the past perfective does not have to refer to a more remote
time than that referred to by the simple past. In some cases, particularly in a
clause introduced by after, the two constructions can be more or less
I ate (T2) my lunch after Sandra had come (T3) back from her shopping. 
I ate (T2) my lunch after Sandra came (T3) back from her shopping. 
After places the eating (T2) after Sandra's return (which we may call T3), so the past, which places T3 before T2, is redundant.
The following pair of examples (from CoGEL) show that the event in the past simple can even be situated before that in the past perfective.
(CoGEL) Adverbials of time position, when used with the past perfective, can identify either T2 or T3. Placed initially, they often identify T2:
When the police arrived, the thieves had run away.
But in final position, the interpretation whereby the adverbial refers to T3 is more likely:
The thieves had run away when the police arrived.
(CoGEL) Note [a] When in the sense of 'immediately after' behaves like after in sentences  and . The following are therefore virtually synonymous:
- I ate my lunch when Sandra had come back from her shopping.
- I ate my lunch when Sandra came back from her shopping.