According to this article on the Marauder's Map, and from the general knowledge acquired through the books and movies, it is quite clear that nothing is hidden from the map. This includes even the invisibility cloak, as this quote from a related question here on SF&F says-
"The number of times I saw James disappearing under it…" said Lupin, waving an impatient hand again. "The point is, even if you're wearing an Invisibility Cloak, you still show up on the Marauder's Map. I watched you cross the grounds and enter Hagrid's hut. Twenty minutes later, you left Hagrid, and set off back toward the castle. But you were now accompanied by somebody else."
My question is- How can a map created by four 15/16 year olds be able to detect the invisibility cloak, when Death wasn't able to detect the third brother hiding under it?
Have I missed a hint somewhere on any indication of some specific magic being used to create the Map, such that it is this special?
Thank you all for posting such good answers. From what I can sum up there seem to be two main points brought to light in the answers:
The Invisibility Cloak was not your average cloak, and the Marauder's Map was an even powerful magical object. It just knows when something is there, regardless of cloak or not.
The Tale of the Three Brothers as told by Beedle the Bard are merely tales. Everything about Death, it's role and presence is probably a complete fabrication.
In light of the above points, my question is: If we agree that Death and it's role in the movie is a complete fabrication, then so is the fact that the Deathly Hallows were given to the three brothers by Death himself(or itself).
Then, who really created the Three Deathly Hallows?
I can recall a quote from Dumbledore in this Harry Potter Wikia link, where he speculates:
"...I think it more likely that the Peverell brothers were simply gifted, dangerous wizards who succeeded in creating those powerful objects" — Albus Dumbledore's speculation
Somehow, I am not fully convinced by Dumbledore's speculation. Besides, he himself thinks it's just "more likely". I also cannot recall any other hint/suggestion elsewhere in the books/movies indicating that the brothers created the Hallows.
Am I missing something (again)?