Battlefield 3 introduced scope glint, a gameplay mechanic added to high power scope attachments. A high power scope is defined as a scope with a zoom greater than 4x. This causes a sniper's scope to be visible from a considerable distance as a bright glare. The glare is brightest when seen from the direct line of fire and is less noticeable at off angles. This effect can be mitigated by either aiming away from the enemy (and using peripheral vision), switching to another weapon, or by switching to a low magnification optic or Iron Sights in customization.
In Battlefield 4, scope glint will only be visible to someone within a 10 degrees viewing angle of the sniper.
In Battlefield 1, scope glint is able to be seen through terrain. It is unknown if this is a bug or an actual feature.