Also, this may imply that the insiders (at least the ones whose actions are observable in this time interval) did not have more than about 18 or so hours notice.

If there is an even earlier group of insiders, the data (try zooming out) might be consistent with one or more of the following from there not being sharp rallies earlier:

• There are too few of them to stand out over the noise of the non-insiders

• They did not get their insider information at the same time and no individual one of them was large or greedy enough to trigger a rally.

• They had automated orders that would be triggered only under certain conditions.