Not Surprising
It was sad news when Madara left the village.
Sad but not unexpected.
Madara wanted to find his brother.
That was understandable, Madara wanted to bring his little brother home.
Tobirama would have done the same for his brothers, all three of them.
Hashirama, Kawarama and Itama. (Too bad that Kawarama and Itama are long dead)
Hashirama wanted to go with Madara, but the man was busy being Hokage. That and Madara had left in the dead of night without telling anyone after he finished recovering. That wasn't surprising at all.
It would be more surprising if Izuna came back to the village on his own willingly and gave up his grudge against the Senju clan. The definition of surprise hints at something being unexpected or highly unlikely and that something occurring anyway.
That battle was over, it was time for Tobirama to continue his experiment once again. This experiment more personal than scientific in nature but it was an experiment all the same.
He couldn't let Hashirama find out about this experiment. Tobirama had to be careful, more so than with his previous experiments. This wasn't going to be easy, knowing what some of the details of this particular experiment might possibly entail.
But it was worth the risk.
He would continue with this experiments.
He would see the loved ones he's lost again, no matter what the cost.
All three of his brothers would be able to be together again.
Tatsuma would return.
And his lover would be united with him at last.
That's why Tobirama couldn't let Hashirama know. His elder brother would certainly try to stop him. And he didn't want to be stopped. Not when this goal was within reach using his knowledge. The cost didn't matter. Tobirama was losing everything at the rate things were going, so he might as well pay the costs while he could... while he could still afford the costs.
After all, anyone would give anything to bring a loved one back, right?