Finder sidebar with tendency to disappear

First of all, I know how to enable the Finder sidebar again, just by switching it on from the Finder preferences.

However, from some strange reason, I've been experiencing that the sidebar has a tendency to disappear on some Finder windows, on at least two of my Macs (one running ML, the other SL). I'm not sure if the Mac running El Capitan suffers it or not.

Since this has been happening since I upgraded one Mac to El Capitan, I'm thinking if it could be the cause, because I sync my files...

Finder sidebar with tendency to disappear