“My cat often goes up onto her hind legs and scratches at glass doors, windows and mirrors. I’m not sure what this means? I’ve opened the door for her to go out, but she doesn’t want to.
I have 2 cats and one of them does it all the time and yet the other one never does it.”
The key to this problem is can the cat see its reflection in each surface? If yes then it could be because she can see herself. Like a budgie with a mirror she does not know it is her and thinks it is another cat. To see if this is the problem you can put some cardboard over the glass and see what she does.
If she simply goes and finds another vertical surface to stretch on does she have a scratching pole that she can scratch at full extension? It is a general rule with cat scratching poles that it must be taller than them at full extension and somewhere on the scratching pole enable them to do so.