For me, playing a human character, it was around 80-82. Before then I could only find them through active searching.
The same was true for the Kotl City hidden boxes, that I used to train up the skill during the summer.
I suddenly went from absolutely no passive detect, to passively detecting everything.
And, I still have plenty of circumstantial evidence that running mounted through Khaldun can cause passive to fail sometimes, during top speed straight runs (the split-second slowing at corners often allows a mounted person to passive in the area, and there's plenty of turns in Khaldun) - but I've had passive detectors run through me while hidden and the hidden chest(s) I was trying to actively detect after passively finding them, without stopping - almost always in the long hallways where the mounted runner was only slowing at the ends. The ones that walked through, always stop, and end up revealing both me and the boxes I was looking for.