Just to answer this old question. The engineering on this device is very tight and even if the volume buttons are in place, they can still be slightly out, or the volume button on the back case may be out. To resolve the issue with the volume up button always being pressed when the phone is reassembled, I removed the volume button from the back case - pull back the white plastic tab slightly and the button comes out. I then used a fine bit of sandpaper to take some of the head from the tip of the volume button plunger, not much. Placed it back on the case, that did the trick and it now works as it should.

There is an inherent fault on the case buttons of the OP3/3T. The Rubber plungers that make contact with the internal ribbon cable button fails over time, it goes hard and brittle, then crumbles. The case buttons need replacing. To do this you require to remove the back case (follow the instructions). Running down either side of the case is a granular adhesive strip. Using tweezers, peel this back carefully until you go past the case buttons. You will see a tiny metal spring clip that tensions the case button in place. Using tweezers again, gently lift that off. You can then push the button from the inside and it will fall out. Then replace with a new button in reverse order.