I thought about Tobirama and SM for a while before. I mean his brother uses it, he shown to know quite a lot about it as well. Maybe he tried to master it before but couldn't perfected it, that would explain why he can sense it like that. After all perfecting SM isn't required to sense NE.Lol. Uchiha tablet teaches them how to use Rinnegan. How do you think Madara learned it :Rolleyes:
He was already a really good sensor so maybe one of Senju's abilities is to sense NE as well