Oh I’m glad they didn’t explain it for sure. The thing is that when you are forced to explain something like how the heck it got there, something is wrong. And the lightsaber could have been easily integrated into the movie in other ways.

For example, Lor San Tekka could have it, presumably to give to Rey when she is ready, but when Kylo cuts him down at the beginning he could take it. Then when Ren is interrogating Rey in the forest, she gets a force vision from Ren, which alerts her to her latent ability. After she decides to free herself in Starkiller base using the Force, she finds the lightsaber alongside Vader’s burned helmet, and takes it. The forest fight could play out as before, except that she has the saber before being thrown against the tree, at which point Finn picks it up. This would also make more sense of Ren’s proclamation that the saber is his.