YouTube must be doing a retroactive copyright crackdown, as over the past several days I have noticed that two of my old videos from a couple years back were flagged for music in the background. It was not music I added, as I don't usually add any. It was playing in the background when I recorded the video. On both videos the music was playing well over 30 seconds, which I have heard is the minimum amount of time a song can play before YouTube's automated ContentID system identifies the song and either labels it or mutes the audio.

Normally, ContentID identifies music immediately after upload, not years later.

I was lucky both times. The record label didn't choose to block the song, which would have earned me a copyright strike. If you get three of those, YouTube will terminate your account.

Because record labels have been known to change their policies regarding copyrighted music, I decided to play it safe and delete both videos. You can read more about YouTube's copyright policy here: YouTube Copyright Center - YouTube