I will make a distinction; didn't Kurama gather a lot of Nature Energy, but also converted it to Sage chakra to send to Naruto? Could it possibly be the reason that Sasuke noticed, because it was chakra and not Nature Energy?

Though @Elve's explanation of Sasuke being able to sense Nature Energy being gathered due to his experience with the Curse Mark makes sense as well

Most people cant feel natural energy but can they feel senjutsu chakra? That might explain the situation

It's likely both. Sasuke had basically mastereed the Curse Mark in the past so that would make sense that he could sense Natural Energy

And he has been shown to easily balance natural energy e.g curse mark susanoo

That's the most logical explanation since what Kurama did is no different than Naruto's instant Sage Mode trick. But, it's been repeatedly stated that it's natural energy. But, then again, sage chakra does contain natural energy as well.

Considering that he didn't sense the natural energy that Kabuto was gathering, the amount gathered definitely plays a crucial role.

Both Tobirama and Minato (in base) were able to sense a TTJ's sage chakra. Tobirama sensed Juubito's while Minato sensed TTJ-Madara's. So, that could indeed explain things.

All Sasuke did was conjure up Susanoo. Juugo's sage chakra simply just adapted to the Susanoo.

Curse mark functioned a little more like Bijuu cloaks more than actual Sage Mode. It replaced the users chakra with Orochimaru's Senjutsu chakra rather than the user taking in and balancing Natural energy

Curse Mark Lvl 2 forces them into Sage Transformation using Oro's Sage Chakra. Sasuke is not a sensor at all to begin with, but he could only sense massive chakras like Naruto using the Kurama chakra as Oro pointed out. So there is nothing saying that if Sasuke WAS a sensor he would not have been able to sense Natural Energy, in fact when Kurama built up a ton of natural energy Sasuke sensed that too. Jugo's DNA is part of the Cursed Seal, Sasuke is compatible with it.

https://naruto.fandom.com/wiki/J%C5%ABgo%27s_Clan%27s_Kekkei_Genkai

https://naruto.fandom.com/wiki/Sage_Transformation

https://naruto.fandom.com/wiki/Orochimaru%27s_Juinjutsu#Transformations

I don't think that it was clear that Sasuke could sense natural energy. If Sasuke could sense Naruto's chakra through dimensions after Hagoromo power-up, then it stands to reason that Sasuke could sense sage chakra build-up. And therefore infer a large amount of NE by deductive reasoning.

Sasuke can only sense Naruto's chakra due to the connection they share from Hagoromo. He couldn't sense Obito, Madara, and Sakura while they were in the Kamui dimension. So, therefore Sasuke can only sense Naruto's chakra. It might be because of this connection, that Sasuke was able to sense Naruto building up sage chakra/natural energy.

So far, these are the theories as to why Sasuke was able to sense Naruto gathering natural energy

• The sheer magnitude of the natural energy gathered
  • Likely

• Experience with the Curse Mark
  • Highly unlikely since Sasuke had no clue that Kabuto was entering Sage Mode and Juugo (the "original") is confirmed to be a non-sensor

• Naruto was actually gathering sage chakra, not natural energy
  • Very likely, since what Kurama did is the "instant Sage Mode trick", which is the instant transfer of sage chakra via clone dispersal. It's also confirmed that non-sages can sense sage chakra as seen when Tobirama and base Minato sensed Juubito's and TTJ-Madara's sage chakra respectively.

• The connection they share from Hagoromo
  • Highly likely, since Sasuke is confirmed to be a non-sensor but, because of this connection, he could sense Naruto across dimensions. If he could do that, then it stands to reason that he could sense Naruto's chakra in general

the senjutsu that kurama built up was literally visible. regular chakra and sage chakra most likely has a different color tone to it. the Sharingan can see chakra and distinguish between different types logically speaking since naruto and sasuke fought along side eachother against obito,madara and kaguya, Sasuke most likely memorized what sage chakra looks like. there is chakra present so its going to look different