Any chance you've installed some other software - I don't know, maybe your display driver or something - that now uses Control-Shift-S to invoke something completely different than a Photoshop Save As?
Does the same thing happen if you're running a different application and you press Control-Shift-S?
Thanks for your reply Noel.
I've just checked and it does indeed do the same in other programs. I hadn't noticed this as I must only use CTRL Shift S in Photoshop.
I have installed a couple of things recently, some add-ons for Visual Studio and mouses without borders... I shall investigate further to find the culprit! Pesky programs!
I know this is a year old, but yours is the only topic I've found on this. Did you find what was making the red bounding box?
@thebundok, I had this same issue and it was due to a mouse and keyboard sharing program called "Mouse Without Borders" it has a hot key for CRTL+SHIFT+S to take a custom screen capture, if you are using this or a similar program like synergy you may want to check this.
@iFixitforyou You fixed it for me! Just like your name says! That's exactly what the problem was. I had no idea Mouse Without Borders had that option. Thanks a bunch!
Thank you for this! Did not even think to look at this!