(Semi-spoiler alert. I've read up to page 247.) Just thinking some more about that Trace. So how did the Death-Eaters track our heroes to Tottenham Court Road, but then not track them after that? It must have been something on the clothes they were wearing. (Recall that Harry had polyjuiced into a pseudo-Weasley, and I think he was wearing borrowed robes.) They changed into muggle street clothes, and may have lost the Trace then.
I'm also wondering whether something muggle-ish will factor into Voldemort's undoing. One of his weak spots is underestimating (or ignoring altogether) things he considers inferior (e.g., house elf magic). Given his utter disdain of muggles, it would be quite ironic if his demise was caused, at least in part, by muggle technology.
It was V's blind-spot thinking to go for Mad-Eye first when looking for the "real" Harry among the decoys. Hagrid was low on his list. Of course, we who know Harry's history knew that Hagrid was top of Dumbledore's list. That's who he entrusted to carry the baby Harry to his aunt's house 16 years ago. Did V not know that, or just not think about it? That would be like him.