You can find out the (highlighted) differences between the in-memory buffer and the on disk version one via "Show Unsaved Changes" on the context menu.
When I halted my evaluation of SL because of this issue I did send them feedback about it, but never got a response. I did have some other far lesser concerns like wasted space if you turned line number off.
Surely not hard to implement (if it's easy to say, it's easy to code, right??)