I want to use them again,
No, for the sake of the humanity, don't do it!
I'm 100% positive it's a practical joke by the engineers of that time; that, or they are outright evil.
The oscilloscope I found this "feature" in the first time had an extremely bright, fluorescent green dot, very well faked to look exactly like the green LED of the same era.
It was the weirdest debugging session of my life. Yes, I started to troubleshoot a scope that turns on, power indicator lights up, but nothing else happens. The issue? Wasn't plugged in. The first and the last time in my life I turned into the typical helpdesk caller moron who spent a lot of energy on an unplugged thing.
Unless you have some enemies you want to play a trick on, don't do it.
It serves absolutely no other technical purpose than to confuse people, and hence, negates all credibility the equipment has. If they are high enough to play a trick like this, what else is there going to be?