Why wouldn't they just use a knife from AK's apartment? Bringing the knife, and bringing it back, seem highly improbable to me.
Jim
Let's suppose that Knox and Sollecito were going to meet Guede to buy some drugs. Let's suppose that this was set up by Knox. That's not outside of the realm of reality. Let's suppose they wanted some good drugs to celebrate the night of the dead - as this event had particular significance for Sollecito. We know that Sollecito's bank account was nearly empty (he was waiting for his father to top it up at the beginning of the month). Meredith had 300 euros of rent money at the cottage, money that could be used to buy more drugs - money that was missing. Reportedly, they were whacked out on drugs in the afternoon. Why not have a drug induced paranoid moment and bring a knife to a meeting with the scary "black" guy so they could get more whacked in the evening?
As for bringing the knife back to Sollecito's apartment, they had no reason to believe that Sollecito's apartment would be searched ... and it's a known fact that murderers like to keep trophies, so why wouldn't a knife collectors that used a knife in a murder keep the knife as a trophy?