The voltage from your batteries is well above the voltage needed to run your unit. You should get an indication from your unit, icon, sound,turns off. This will happen before the batteries get low enough to make a difference. Why, the detector has a voltage regulator that keeps the voltages steady and as long as the batteries are above a certain point you are OK. Your manual may cover this. No matter what anyone says almost all modern detectors will go to the level that is stated in the manual. Earphones will help save your batteries only if you can adjust the sound from your detector. If you adjust sound at the headphones the detector is still amplifying the signal to the speaker and still uses the same amount of power.......................................................... Man that was long!
Ed