Oo--Oo--Oo--
I just remembered something.

In chapter eleven, last page, Mundungus said,
"I was selling in Diagon Alley and she come up...she took a fancy to the locket an' told me she'd take it and let me off that time, and to fink meself lucky."

How did she know?

Then again, Voldemort himself didn't suspect that it was missing until page 549, when a Gringots goblin told him that the cup had been stolen.