I *like* guessing.

[darlingfox]

So, the Horcruxes. Let's see what we have and may have.

 

1. The diary. Chosen because it proved Voldemort was the heir of Slytherin. Destroyed by Harry in CoS.

2. Slytherin's ring. Destroyed by Dumbledore just before HBP.

3. Hufflepuff's cup. Stolen from an old wtch when Voldemort was younger. Intact.

4. Something from Ravenclaw or Gryffindor. According to Dumbledore, the sword isn't one. It's possible that Luna can guess where it is. Intact.

5. The locket. Might be either Ravenclaw's or Gryffindor's. Probably either stolen by Mundungus or still in Grimmauld Place. You really didn't think JKR would mention "a locket they found while cleaning that nobody could open" in OotP just for fun? Intact.

6. Nagini. Underlines the Slytherin connection. Intact.

7. Still inside Voldemort. Intact.

 

2 destroyed.

2 in places Harry knows.

3 lost, one of which may still be in Grimmauld Place.

 

Apparently Harry isn't one, although there's still a chance he may be number 4.

 

If Harry'll realize one may be in Grimmauld Place/Mundungus knows where it is, that leaves only 2 to really search for in the last book. And that means they'll probably go to Hogwarts if it's still possible. Luna has weird theories and perhaps she knows how to find one. So, the only thing they have no glue how to find is Hufflepuff's cup which is probably in a very obvious place. Voldemort isn't all that subtle.