I absolutely remember that There is an npc in a car that smiles at you if get the egg while undyne Is bench pressing Cars but I can't find any info on the wiki does It actually exist?
It does. If you get the egg in chapter one, if you look in one of the cars (not including the one with the sanil in it, or the one with the scared donut guy in it), there is flavor text about there being a man that waves at you, but only once.
https://deltarune.wiki/w/Man#Chapter_2
If the Forgotten Man was met during Chapter 1, he appears in Hometown at the beginning of Chapter 2. When leaving the School to go to the Library, inspecting any car in the traffic points out that there is a Man in it who waves happily at Kris. Inspecting a car once more reveals nothing about the Forgotten Man.