Multiple times I am listening to the daily mix from Spotify or listening to random songs that play after my own playlists finish and I am listening to new or old songs that I enjoy but there are times I forget to add them to my own playlist or attempt to find it at a later date but sadly can not find. So my idea is that anyone who is listening to a random song for longer than 70% + of the song I think it should automatically go into a new playlist so you can listen to your new favourite songs again?
Also, I'd recommend you to open a last.fm account. It's a great way to keep track of what you listen in Spotify, and most other music players. You just need to link your Spotify account within your last.fm account preferences, and you'll log what you listen to. With this, you'll be able to see what you were listening at any point in time. Very useful!