I decided to open up my Pixel again to try things out and I found the cause. I simply glued that tiny little hole on my screen when I assembled phone before. Wow. I didn't take possibility of gluing proximity sensor to be the cause, since it did react to mirror but somehow that was it. Maybe there was a little gap that allowed tiny beam of light to go through to it. But either way I'm happy it's fixed, because those past couple of months very really annoying: touchscreen was working while in my pocket almost dialing emergency phone couple of times; with every call I made or took I had to lock screen manually. You don't really appreciate those little things only until they're gone :)