If the valve is closed at the bottle behind F/O's seat, you may still get brief flow, and the needle will drop. This can happen if it has only recently been turned off and there is still residual pressure in the lines. You will get indication on the guage which will drop when you hit the test button on the mask. In order to make sure you have fully checked the system, ensure the valve is open, you have pressure showing, and it doesn't drop (or returns to original indicated pressure) on releasing the test button. Before carrying out the test, have the intercom button on the relevant side depressed, and audio switched to speaker. When you then press the test button you should hear the "whoosh" of oxygen through the speaker. You have now also tested the microphone in the mask, and that mic input switches auto satis.