This question led me to asking this one. R.A.B. takes the "real" locket out and asks Kreacher to destroy it, then puts the "fake" locket back in the basin. Why would he feel the need to do this? He knows already that Voldemort has used someone else to drink the potion (Kreacher). In fact Kreacher's story to him about his mistreatment on the island seems to push him over the edge to act.
Instead of assuring that someone else (most likely innocent like Kreacher) would have to drink from the basin, why not just leave the note outside the basin for someone to find? If Voldemort finds it then so be it, at least then no presumably innocent person would have to drink again.