The camera, light and object selection lines are suddenly appearing in my rendered video's. Some cameras and light symbols are set in 'conditions' to 'show', but they they didn't show up in rendered video in the past. Now they do unless I change conditions to 'hide'. Even the 'selected' object frame is rendering. 

Is there a setting I may have accidentally enabled, or does this sound like a bug?

In your Render settings make sure you select "Final Render", not "Preview".

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76% of original size (was 661x19) - Click to enlarge

I'm likely wrong but I could almost swear the objects didn't render in 'preview' in the past. I mostly use 'preview' for the final videos since they are going on social media / YTube. If submitting to film fest I'd used final render, which takes about 5 times longer on my machine (no graphic accelerator). I guess I have to get in the habit of 'hiding' them.