Spotify Mobile playing songs NOT on spotify

[perrydrums]

Hi,

I've noticed something interesting on Spotify for Android (and maybe iOS as well):

On my desktop I have downloaded a couple songs of YouTube and those are placed in a Music folder. I've added this folder to Spotify so I can play those songs through spotify, having all my music in the same place.

Now, when I shuffle that playlist on my phone, there's a change that song gets played back! There's no albumcover or artist linked to it. The lockscreen background stays whatever it was from the previous track. It does play the entire song though! I can't find the song anywhere on Spotify, I'm pretty sure it's not on it. 

I can only listen to that track by change, if it happens to play using shuffle. I can't tap on the song in the playlist.

How is Spotify mobile able to play that song back to me? Does it save my personal tracks online as well? That shouldn't happen, right? And if it should, why can't I play that song whenever I want than?

Has anyone experienced this as well?