Hey Hobbs, I have a few things to add to this. LOS issues will make or break you in this game.
a. Sometimes, the enemy will be spotting you, and you don't even realize it. That would explain how he's attacking you in the same turn from your "hidden" position.
b. Sometimes, people refuse to watch the replay, and during the replay, a jeep (or whatever) moved up to your hidden unit, revealed its location, and moved away. Always be sure to watch the replay.
c. Sometimes, the enemy will blind-fire their artillery into your hidden unit's hex. Rockets are especially devastating when used for blind shelling. I remember one time KK Rossokowski's rockets hit my unspotted trucks (he likes blind-shelling too :stir:), and I checked the details - a whopping 18 attack factors were going into the attack, not the usual two or three. If your unit retreats as a result of this into enemy los, it will become spotted.
d. Last but not least, some players in high-visibility scenarios will not "zoom-out" enough. I often switch from view 1 to view 4, to make sure distant hilltops can't see that hex I may be moving my loaded trucks into. The scenario "Hedgerow Inferno" is a perfect example of a scen where you could swear that nothing can see your hidey-hole, when in fact, an OFF-SCREEN hex far away can see you just fine. Of course with my luck, that's the hex with an enemy spotter in it.
Hope this helps.