Hidden Packages?

Yes, there are. First, finish all possible side missions (marked by yellow circles on map), then if there are still unconquered territories, surely there's a hidden package side mission. Just kill enemies within that unconquered territory until one fallen enemy activates that side mission (don't let the enemies die falling, near water or die frozen... just to be sure)

A few of the regular side missions on the first island don't show up until you've been on the second island for a bit. But there are a few of the hidden package ones from enemies. There is also another one when you kill an enemy where you have to find and kill a convoy they were supposed to join.