Excellent info Thanks all. Now I've got another question since I've never run into this before.

Is there some kind of time limit that the game sets for the missions? I was flying a close air support mission and after I had completed my mission, I flew around for a while and destroyed all of the SAM's and armour on the uppper half of the map. Without anymore targets, I flew towards my FARP as fast as I could. I wanted to see how fast the apache could fly. I had been flying for a little over an hour at this point.

I had my FCR in ground mode and it was actively scanning just in case I had missed anything. ASE was showing nothing. All of a sudden, my gunner yells out that there's a fast mover at our 7 o'clock. In the second it took me to switch from ground to air mode for the FCR, I'm shot down. I had never seen any jets or heard any reports of jets for the whole mission and it seemed really weird to be encountering one so late and so suddenly.

Does the game send out jet's if you're being a dick :p Seriously tho', is there some point during a mission where the game begins to ratchet things up to try and give you a "gentle" reminder that you should get the hell back to your FARP? I wouldn't think so since this is a sim and not an arade game.

Enemy airstrikes are the same as with your own artillery and airstrikes you call in. The mission designer chooses the amount of artillery and airstrikes available to both sides individually.
The artillery shells are spawned at the impact-area (launched from imaginary launchers) and the planes are spawned just outside the map.
In campaign-mode I'm guessing the amount of airstrikes the enemy has depends on many factors. I think one of them is if you destroy all grounded frogfoots in the previous mission the enemy will have very few or no airstrikes available in the next. The grounded (bunkered in) Frogfoots usually are at the edge of the map on the east side, far from any of your target areas.

Then how the heck do you avoid being shot down by a jet on your way home if you get so little advance warning? Do you fly NOE all the way back too even though the air defenses have all been knocked out? I can't speak for anyone else but flying NOE is pretty tough. Flying at 30ft doesn't give you a lot of room to work with. Guess I'll have to keep my radar on during the trip home =/

I usually fly with the A-radar on 360 on my way home and I make sure I fly over area I've cleared earlier. I don't fly NOE more than I have to, so no, I don't fly NOE on my way home unless the situation requires it. I rather fly fast (140kts) and semi-low(~100ft AGL) on my way home rather than low (~30ft) and slow(50-70kts).
I figured if it takes me 5-10 minutes to get home instead of half an hour its less of a chance I'm going to encounter any fast-movers.

Usually I hear a report from a SAM battery calling out enemy air contacts, which cues me to take cover behind a steep hill and turn air radar on. If I still have Stinger and time it right, I can pop a fast mover as he zooms out over the hill, acting like he's hot stuff. Complicates things if you don't have any Stingers (which is usually the case since all four were spoofed by that Havoc's flares.)