I'm familiar with the plus-que-parfait and its anteriority. If X is a reference point in the past, then the plus-que-parfait verb took place further back than X.
However, I sometimes hear it without any obvious anteriority. A common example seems to be « j'avais pensé », which I feel should be « je pensais » :
J'avais pensé que tu viendrais aujourd'hui.
I see that I'm not the only one to be confused by this; here's a relevant WordReference thread.
The last poster there says this is because the X reference point (« ... avant de me rendre compte que ... ») is implied. But surely that would legitimate the plus-que-parfait all over the place where there's no obvious grammatical reason to use it! Can't we find invisible clauses anywhere we look?
Is this "non-anterior PQP" standard, or familiar or mistaken (from a prescriptivist point of view)?
If it's legitimate, is it isolated to « j'avais pensé » and/or some other small class of verbs, or can you do this with any verb?
As always, is there something obvious I'm missing?