Good call. I'm assuming you heard the tidbits from some random GOP consultant and Jeff Flake? They both said - being serious - that that 30-35 Republicans would vote to remove if it was a secret ballet.
Here's an article making the case for a secret ballot back in '99. I bet we'll see more discussion of this going forward.
I don't see it as 100% for acquittal though. It's much more precarious than that and Trump's behavior is too unpredictable. Support for impeachment is about 50/43 right now. And that's with a behind-closed-doors investigative process. If you get 70-80% of independent voters supporting, pulling the national support to 60% during the trial - things could get interesting. National support will not be based solely on the facts of the case, but on Trump's behavior during the process So everything he does to secure and rally his base risks persuadable voters turning on him.
I would guess that those 30-35 Republicans are going to be highly influenced by public opinion. They know what they should do, but do they have the political cover to do it? Most Senate Republicans, IMO, would love for the era of Trump to be over. They don't like where it's taking the party and in many ways they're held hostage to it. They realize the danger the party is in running an incumbent who was just impeached, and they may fear what he'll do after being emboldened by surviving the trial.
I think this is much closer to 50/50 than people realize if you factor in the way Trump is likely to act going forward and during the trial. Look at the op-ed McRaven wrote last week. How does Trump respond if a bunch of retired Admirals and Generals start writing op-eds during the trial, calling for removal? The snowball that could build is hard to predict here. Imagine what he'll do if at some point Mitch says he's in real danger of being removed. He'll go bonkers, looking more unfit and building more support against himself.