I just realized Voldemort probably cast the revised Horcrux on Roger Bacon’s diary with wordless magic:
The book vanished back into Professor Quirrell’s robes, and he rose up from his chair. Professor Quirrell started to walk over to the door -
and staggered, suddenly lurching into the wall.
“It’s all right,” said Professor Quirrell’s voice, which suddenly sounded a lot weaker than usual. “Sit down, Mr. Potter, it’s just a dizzy spell. Sit down.”
[...]
“That, Harry Potter, is the diary of Roger Bacon.”
Harry almost fainted.
Nestled up against the wall, where Professor Quirrell had stumbled, glistened the crushed remains of a beautiful blue beetle.
Or he could have just cast the spell at any other time of his choosing prior to giving Harry the diary. For that matter, he could have not done it at all, since there’s no actual benefit to making the diary a Horcrux—he already has all the Horcruxes he needs, and it’s not like he can possess Harry anyway.
I just realized Voldemort probably cast the revised Horcrux on Roger Bacon’s diary with wordless magic:
[...]
http://hpmor.com/chapter/26
Or he could have just cast the spell at any other time of his choosing prior to giving Harry the diary. For that matter, he could have not done it at all, since there’s no actual benefit to making the diary a Horcrux—he already has all the Horcruxes he needs, and it’s not like he can possess Harry anyway.
Turns out I was quite wrong anyway.