In modern warfare, why can telescopes mark enemies with red frames?

The "telescope" you mentioned is not actually a telescope in the true sense. The telescope that people generally talk about is just an optical observation instrument.

The "telescope" in "Modern Warfare" is actually a telephoto lens under the head of an unmanned reconnaissance/attack aircraft.

The so-called "marking enemies with red boxes" is actually infrared recognition technology.

In many movies and news pictures, tank turrets in the United States, Israel, and the United Kingdom have black or white "V" marks on them. This "V" is not a slogan or a hull number, but a hull number. It is an infrared identification mark. When one's own aircraft or missiles want to attack a tank on the ground, the infrared reflective mark will be recognized, and the operators of the aircraft and missiles can avoid accidentally injuring their own people. "Marking the enemy with a red frame" is actually an identification technology, but it only identifies vehicles and personnel with weapons and no own identification marks, so they are probably the enemy!