In chapter 79, Snape destroys evidence of any notes from S in Hermione's room. Before doing so, he asks this:
"I shall go look for them," Severus said. "And then, I suppose, start looking for You-Know-Who." A frown crossed the Potions Master's face. "A task at which I haven't the faintest idea of where to start. Do you know of any magics to find a soul, Headmaster?"
Now, who else do we know who wants to find bits of souls and would delight in using Dumbledore to do so?
It seems not unlikely that Voldemort has polyjuiced or otherwise disguised himself as Snape (a Time Turner allows him to be here while also testifying at the ministry). If this is true, then it is Voldemort, not Snape, who now has some of Harry's hair. The trial is a superb way by to distract Dumbledore, McGonagall, and HJPEV while he tracks down Horcruxes or the Resurrection Stone.
(It is also possible that Dumbledore has cottoned on to him - hence the phoenix, to be ready to depart at an instant's alarm.)
Forgive me if any of this has already been mentioned; if it has, I must have inexplicably missed it throughout my perusals. But is there something relevant, perhaps obvious, that I've missed related to this hypothesis?
I should have been clearer: it is only beneficial for him to ask. Option 1: he knows where they all are, and in this case he finds out whether Dumbledore knows as well. Option 2: he doesn't know where they are, and has resorted to things like asking HPJEV how he would hide things, with the goal of narrowing down things he's forgotten.
But I see your points. Thank you for helping me de-muddify my thoughts.