You have to clear a Glock before you can disassemble and clean. You have to pull the trigger before removing the upper. Well, at least on a Glock 26, which is the only one I’ve handled (I assume they are all similarly operated).
The loaded chamber indicator on the Glock I’ve handled is actually terrible. It is hard to distinguish flush from protruded. I just pull back the slide and take a look, and then take another look. And then, I don’t point it at a kid! :)
I was actually referring to the kind of ECI that just consists of a yellow or orange length of plastic that you can push down the handgun barrel from the muzzle end, through the chamber, and into the breech. Cheap, but the best kind of ECI, IMO.
There’s no way a cartridge is in the chamber if that is showing through the ejection port.