Yes, you were... :)
[trough] seems to be the opposite of [peak], and does not seem to function as the opposite of [past], because -besides outputting different things- (the doc means that) it continuously updates the "threshold", but a constant one is needed in this case...
... I think.
You're right. I was responding off the top of my head without reading very carefully or giving it much thought (which was not a great idea), and you're correct that trough is the opposite of peak, not past. I like big_pause's solution, which is economical and seems pretty problem-free.