Hi Asphodel.
I don't think he could have created a horcrux when he was in DD's office as you have to kill someone and cast a spell at the same time.
Now I'm thinking on it and seeing as I've just read HBP again I am now more and more convinced that LV hid one of his horcruxes in the room of requirement...here's my theory.
LV used to work in Borgin and Burkes where there was the other vanishing cabinet. The vanishing cabinet in the room of requirement was broken for years until Draco fixed it.
Draco fixed it and there was a passageway between the two. That means Tom Riddle could have been using that passageway for the whole time he was working at Borgin and Burkes. We know it had been broken for ages but LV could have done that deliberately. At the time he was working for Borgin and Burkes he acquired the Hufflepuff cup so he could have hidden it in the Room of Requirement which is already protected by enchantments - and Hogwarts we know is a place which LV/TR regarded highly as it felt like home to him.
The thing I find curious is that JKR mentions a heavily blood stained axe in the paragraph when Harry is looking around the room for a place to hide his potions book. Why does she mention the axe? What significance does it have. Is it the axe that was used to take out Nearly Headless Nick. This would be something that kind of related to Gryffindor...
Any thoughts anyone? Or am I just being a lunatic!!