Ditto. Or even an old-school LED that indicates data transmission like the HDD LED in old computer cases. Make it in hardware so that if any data is sent through the wire, it lights up.
Old? I'll admit I haven't changed my case in a couple years, but mine currently has one, and I can't remember the last time I've used a desktop computer that didn't have one.
https://www.thefreedictionary.com/old
Old doesn't have to mean extinct :)
[0] Guri et al, LED-it-GO
It seems almost quaint to worry about this when so much software has web spyware baked into it -- but maybe we need a virtual spyware led that lights up every time something talks to Facebook/Google/etc tracking. The light would be blinking non-stop for many people.