Red Mushroom House in World 4

I think you are mistaken -- using a warp whistle while in the warp island should always take you to World 8. The Super Mario Wiki confirms this:

If a second whistle is used while in the warp zone, then the player is taken directly to the World 8 Pipe.

There are also two warp whistles in World 1, as well as the third one in World 2. One is obtained in stage 1-3 by holding down on the D-Pad while standing on a white platform near the end of the stage. Another is obtained by using Raccoon Mario to float on over the top of the end of the Fortress stage in World 1. And the last is obtained by using the hammer item to break a rock in the top-right of the map of World 2. A video showing their locations is here: Super Mario Bros. 3 Warp Whistles

There were two hints that the warp whistle was there, and both of them were contained in letters from Princess Peach.

Between worlds 3 and 4 you get this message:

The White Block contains magic powers that will enable you to defeat your enemies.

And between worlds 6 and 7 you get this one:

I am well. Please retrieve the Magic Whistle hidden in the darkness at the end of the third world

Although it's not a map and a sign, both hints together are enough to point the way to this particular whistle at 1-3. It looks like a basic way to reward a replay as you made it through the game. There are even less clues for the other whistles; one other letter makes reference to "east of the sand dunes" for the desert world whistle, and for the first fortress whistle, there's nothing to hint at it other than Super Mario Bros hid a warp zone in exactly the same way (plus as mentioned by Ryan, it was practically advertised in a movie).

Otherwise, intentional vagueness and cryptically hidden random-access secrets (like white mushroom house, and pirate ships full of money) was a good way to sell video game guides in the early Nintendo days.