Yeah, the tracker thing always made sense in theory but not in execution. I don't get how Johanna and the other rebels thought that this little jaunt would go unnoticed by the Gamemakers. Wouldn't it become instantly obvious that the tracker had left Katniss' body? Since it looked like Johanna was digging in there pretty deliberately, they would know that something was up. And it wasn't like Katniss instantly shot down the field afterwards; it was at least 10-15 minutes later; maybe longer. That would been more than enough time for the Capitol to be able to detect Thirteen's hovercraft in the area.
Anyway, the *only* explanation that I can come up with is that maybe trackers did go missing every now and then and the Gamemakers just dealt with it; I mean, what if a tribute hacks of another tribute's arm? No more tracker. They could have thought that Johanna did it on accident while she cut Katniss but yeah, it's a long shot at best.
Can I just say, though, how much I am looking forward to that crazy arena in CF? It's going to be crazy cool. Plus all the tributes except Peeta and Katniss will be adults, so there will be a completely different vibe. I think Johanna will be around 20 and we already know that Finnick is 24 (he won 10 years prior at the age of 14). And everyone else besides them is older.