Why do I have icons showing in the upper right hand corner on some of my thumbnails in Bridge?

Hi, @musicians dogsbody, let me back up to some basics: Adobe Bridge CANNOT edit an image. It's a viewer, and that's it. Any adjustments that you do "from" Bridge is probably done with the plug in: Adobe Camera Raw (ACR). 

Let me show you.

I've got a JPG image here, I've made a duplicate of the image. I'll open one in adobe Camera Raw

and I'll darken the image so that it's different from before.

Here's a screenshot of the top of the window I'm doing the adjustment from. Notice it says "Camera Raw."

In the ACR window, I click "Done" on the lower right corner, because I'm finished making adjustments. 

I've taken the other image, opened it directly into PS and darkened it as well. I close and save the file.

Now, when I look at the two images, side-by-side, both are darker than they were but one has the icon you're concerned about. THAT one was the one adjusted in ACR. The one of the left was adjusted in PS. 

Now, here's the BIG difference. When I closed and saved the PS version, I "burnt" the changes into the image. I cannot go back, I cannot make subsequent changes, that image will ALWAYS be that way (too dark).

But the image with the icon shows that it was adjusted in ACR. I can right-click on that image and select a variety of the Develop settings. For example, I can copy those settings, select a bunch of other images in Bridge and apply those settings to them. It also lets me know I can double-click that image, open it directly into ACR, and completely undo any change, or make aditional adjustments becuase the file was NEVER changed, only it's "interpretation" was changed. 

 Lastly, I can select "Clear Settings," and easily remove any change that was made. 

A couple of more things: JPG and TIF file formats are container formats. That is, they sort of work like folders. The changes made in ACR are contained in an XMP file that is a separate file and are usually called "side-car files." In the case of JPG and TIF files, these containe the XMP file so you never see them. One caveot to this is if you start to do Masking in ACR (and you should check this amazing tool capability out, it's big), this creats ".acr" files that are not contained in JPG files. If you get these, be sure to keep the image file and the .acr file in the same folder.

Raw files from cameras cannot contain XMP files, so there, you do see these side-car files. However, if you convert the raw files from your camera to the DNG (Digital Negative) format, you are back to container files. FWIW, I've always made this conversion.

One last point: it takes Adobe software to read Adobe results. That is, if you take a JPG file, make adjustments to it in ACR, think it's wonderful and upload that to social media or to a friend. They are likely to see the file BEFORE you made any adjustment to the file. What you must do before sending the file is to save a copy (to burn in all of the adjustments), and send off the copy. 

I hope this makes sense, ask away if you need any extra help.