Yes, Virginia, there is a traitor

[pippin_999]

No: HPFGUIDX 159262

I've been thinking about Draco's movements on the night of the Tower, and
I've noticed a couple of curious things.

One, JKR  shows us in OOP that the Room of Requirement can be
locked from inside, presumably keeping anyone else from entering even if
they require it in its current form. Yet Draco must have  left it unlocked, 
because Trelawney got in after him. Why? Was Draco expecting someone?

Two, after he threw Trelawney out, Draco remained in the RoR until
the DE's arrived. Ginny says it was  about an hour after they started
to keep watch. According to PoA, it takes an hour to walk to Hogsmeade.
So, just moments after Draco heard from Rosmerta, the DE's
arrived and the attack began. We know Voldemort can
summon his servants instantly. So far, so good. But--

If Draco didn't know  until then that Dumbledore would be
out of the castle that night, why wait for an hour in the RoR?
Wouldn't he have gotten out  of there in case Trelawney came back?

The Order knew that Dumbledore would be gone that night. Did one
of them tell Draco to stay put?

Pippin