This game mechanic is quite depressingly frustrating. Having rebels in "hiding" means they do not add to Advents "Alert Level". Advent tries to raise advent strength to the alert level in region that is generated by missions and rebels doing anything else but hiding. I too was working under the assumption, that rebels actively hiding will mask the other activities rebels are persuing. This is not the case.

While the game proceeds, expect advent strength to go up everywhere. The only way to reduce advent strenght is to play "reduce advent forces" or "Supply Run" mission. Supply runs stop advent from moving forces to a region, while assaulting advent forces actively reduces them in the region, but you will have to use many rebels on intel (7 - 9) to detect those missions early enough to have a chance of winning those missions, which raises advent strength. Its devious really.

To slow thie rise of (overall) advent strength you therefore have to set rebels on intel. Hiding everybody and stopping activity in a region will only result in advent moving somewhere else.

I stopped gathering ressources in regions not liberatet. This is a honey trap IMO. If you go for ressource gathering, the game will punish you with more difficult mission, so you need more ressources to win, and so on.

The missions that reduce advent strength also give You ressources though. So instead of gathering them, I take the resources from advent.

The game mechanic is described here:
http://www.ufopaedia.org/index.php/Advent%27s_Agenda_(LW2)

Many thanks