[Spoilers] No one noticed!!!

[Draxus]

The fact Minato could sense Naruto has more to do with the massive amount of chakra he and Kurama have and less to do with Minato's sensing ability.

No one is sensing those 10000 ninja you keep referencing. Just Madara and Naruto/Kurama.

 

[GaaratheFifth]

Naruto and Kyuubi's chakra dwarf everything around them other than Madara and 8 tails.

Still, he is definitely a perceptive ninja.