Ok of course it's impossible but I wonder what can be done so a cat can always get in the house or at least let you know it's outside? I cannot make a cat door unfortunately. Are there any options for a "cat bell"?
My cat scratches doors like a dog when he wants in (he doesn't go outside alone but will do it if we close the door to the bathroom when he needs to use it or just close the bedroom door) But we've never taught him to do it and frankly it can get really annoying !!! What about putting a collar with a strong bell on him ? so you can hear him when he's near the door? I had a cat that I had my bedroom window cracked open and he would go out and in through the window.. he would also scratch or meow if I forget to leave it open
I'm concerned about not hearing the bell but of course it's better than nothing. Thanks for your reply!
I taught my cat to tell me when he wants to go out by putting a bell on the door knob. It takes a bit of training and reinforcing the sound whenever he goes outside. Now he doesn't scratch on the doormat when he wants to go out. I tried putting a bell on the outside door knob to let him come in but it doesn't make enough sound to get our attention so he doesn't use it.

You can clicker train cats to do things that you want! I suggest looking into a the book The Cat Whisperer!!