'Cleaning up leaked objects'

I’m getting this a lot, and despite searching, have no idea what it is or how to fix it. Can anyone give me an idiots guide to what it is and how I can go about preventing it?

Cheers,

SB

It’s nothing serious.

As I understand it, it used to be displayed in an early version, then the message was removed/hidden for a few versions, then seemed to come back in 3.2 I think (possibly 3.1).