Let's address each of your examples in turn:
First year: "Hey, Voldy's balancing on Quirrel's head."
We don't know that Voldemort would have even showed up on the map while possessing Quirrel. Additionally, even if he did show up on the map he might have appeared under the name Tom (Marvolo) Riddle which Fred and George probably wouldn't have recognized.
Second year: "Hey, our sister is going somewhere that doesn’t exist."
They probably wouldn't know that she was going somewhere that doesn't exist. The Chamber of Secrets wasn't shown on the map, so Ginny just would have been missing during the times that she was in the Chamber of Secrets. Fred and George would only notice this if they were specifically looking for her and while she was in the Chamber. And even then not being able to find her might not have been so suspicious, given that they might assume that they just couldn't find her among all the other names floating around the castle.
Third year: "Hey, some dead guy and a mass murderer are under the whomping willow, along with our teacher Snape, Harry, Hermione and our brother."
First of all, Fred and George no longer had the map at this point. They had given it to Harry earlier in the year to enable him to sneak into Hogsmeade. Furthermore, even if they had had the map, and even if they had been looking at it precisely during the altercation in the Shrieking Shack, there wouldn't have been anything to see since the map does not extend that far. As stated in Chapter Seventeen of Prisoner of Azkaban:
"Where does this tunnel come out?" Hermione asked breathlessly from behind him.
"I don't know... It's marked on the Marauder's Map but Fred and George said no one's ever gotten into it.... It goes off the edge of the map, but it looked like it was heading for Hogsmeade..."
Fourth year: "Looks like Voldy's in a maze, with the champions."
Again, Fred and George no longer had the map at this point. Moreover, Voldy was not in the maze. Voldy was in a graveyard where Harry and Cedric were transported via portkey.
Fifth year: "Hey, Harry’s on the train, it looks like he’s stunned."
I assume you are referring to the time that Malfoy used the full Body-Bind curse on Harry in Chapter Seven of Half-Blood Prince (Sixth Year, not Fifth Year). Once again, Fred and George did not have the map anymore. In fact they weren't even students at Hogwarts anymore, having left towards the end of the previous year. Additionally, the train doesn't ever enter the Hogwarts grounds. As described in the same chapter:
Having always traveled there by carriage, Harry had never before appreciated just how far Hogwarts was from Hogsmeade Station.
Thus, even if Fred and George had had the map at the time, and been looking at it at that precise time, there would have been nothing to see since Harry was off the map.
In conclusion, there seem to be perfectly reasonable explanations for each of your examples, so these examples do not indicate that the twins don't notice anything.
