its obvious the game isn't intended for it, but it is still possible to at least do some of it stealth. Usually I will sneak into a base, plant a bunch of triggered explosives, and collect as many upgrade parts as I can without being detected, then when I do get detected i set off the explosives, also sometimes I will scout out a base picking off the guards with a sniper rifle, and sometimes I am able to take out everyone without being detected, then I can go into the base freely and pick u[/destroy everything.
Also by detected I don't mean the heat stays at zero. As soon as you kill an enemy or destroy something you automatically get a tiny bit of heat. but that just means the enemy will spot you on sight. its not until they actually start shooting at you that you've been detected.
Also sometimes a lone soldier will find me and start shooting at me, and after I take him out it seems like none of the other soldiers know I am there
"Apparently the phrase "mile high club" equals sex to some and COD to others lol" -wretchedsmurf
I might have to give this a try. So my standard method is to find a chopper with missiles, take out anything that can be blown up (and kill any enemy choppers that show up) and then mow down all of the infantry that runs out. Then I land or parachute in, find a mini gun and dismount it to cut down the stragglers that somehow survived the chopper while I pick up all the crates.