Being seen by the enemy is usually not a good thing. The less you move, and the lower you are, the better chance of hitting the enemy you will have. Try not to stand up or be crouched too much, it could lead to death or injury. If enemy armour shows up and you have no AT weaponry, then either run away from it, or start sneaking around.
If you are sneaking at night, never stay at a peak of a hill or mountain, it will create a silhouette making you easier to spot.