sinkhole,
A couple of items that might help. GV's won't have icons on each other, only friendly icons, so he couldn't have seen you that way. One of the key issues with GV"s though is spotting your enemy first and getting off the first shots so you really need to have good situational awareness. For that, I'd first consider making sure your detail levels are set a bit higher, there is a "ground-detail" mode (SHIFT-F4 I believe) that you can turn on that will help. I find that when I was playing with detail set very low (because I had a PC with little horsepower) I'd get things shooting at me that I couldn't see AND there would be lots of trees/buildings/etc that wouldn't even appear until I was almost on top of them. That might explain why you had trouble seeing him if your detail was set low. Secondly, the GV game really requires great patience and observation. Guys will sit and wait for you with their engine off, listening as much as looking. they can hear you coming and set themselves up to spot you very quickly. It's an acquired skill. They'll wait until you are very close range and pop you with the first or second shot.
So, some tips:
1) stop, turn off your engine, and listen sometimes. Dont' just sit there with the engine idling lest they hear you and spot you.
2) the best place to drive from is actually the roof-commanders spot. Get moving and then switch to the top (usually position 3) so you can look around. You can still steer with the rudders from up there (if you have them, or a twisty stick).
3) set a "comfortable" zoom level in your gunsight. You want to be consistent because after a while you will develop a "ok, his vehicle is the size that I'd say is D1.2 away". It's amazing how quickly some people can estimate range because they simply know how large a GV looks at different distances. If you vary your zoom level a lot then it's hard to develop that.
Finally, one more point, aim for the turret or drivers position on most GV's as that is the part that will lead you to a kill. If you kill the turret he can't shoot back, but the drivers position tends to give kills. Someone posted some pics a while ago showing that. Oh, and the M8 is fun because of it's speed, but it's not really a good match for real tanks. The gun is only effective at short ranges and the armour is paper thin against even .50 cal hits.
Hope that helps.
-Soda