Well, I think it’s certain to say that right now nobody but Rumple knows about the dagger. He never told a soul and the fact that Regina told Belle to kiss him to break his curse supports that theory.
The Cane idea sounds great (reminds me of Lucius Malfoy’s wand in the Harry Potter films), but I think the knife is too broad to fit in the cane.
I saw some knifes in the shop, but I doubt he would keep such an important object there (Then again, sometimes hiding things in plain sight is more effective …). So it either is in his house – in a safe maybe – or he may have buried it in the forrest! We saw him digging in “The Heart is a Lonely Hunter”, so maybe he hid the knife there (I actually doubt that, too, but hey, who knows.)?