A member of the Paradise Lost Army resistance group in Eastern Europe was active during the Guns of the Patriots Incident. When the country was placed under curfew by Raven Sword, the PMC stationed in the area and part of the mother company Outer Heaven, the resistance member frequently walked at night to meet with the resistance while whistling a distinctive tune. Solid Snake, who was trying to find Big Mama, the leader of the resistance group, proceeded to stalk the resistance member over to their headquarters in such a manner that he didn't even sense him at all. Upon arrival, he then was forced into a Human shield position by Snake as the latter forced himself into the hideout. He then confirmed to his fellow resistance members that he didn't even sense him coming, meaning he had to be the person they were waiting for.
Behind the scenes
The stalked resistance member was a key character in the third act for Metal Gear Solid 4: Guns of the Patriots, Third Sun, with stalking him without detection being mandatory for completing the mission. The song he was whistling was the MGS4 Victory theme, a remix of the Metal Gear Solid Main Theme.
The idea of stalking a whistling soldier while trying to reach a particular objective was considered as early as the development of Metal Gear 2: Solid Snake, where Snake originally had to stalk a Zanzibar Land soldier whistling Theme of Tara to find Dr. Pettrovich's cell (this was replaced in the final version with tailing a soldier wearing a green beret).