Have you noticed that when you are traveling in electric trains like suburban trains, your phone calibration gets disturbed due to electromagnetic noise and your phone prompts you to recalibrate in you are using an app that needs compass. This is just one instance, there are many which can disturb compass calibration during traveling. If my car garage is open and a car passes by, garage lights turn on by sensing the radiated EM noise. If you are near a power transformer, your compass goes cranky.
In short there is nothing wrong in checking if all is ok or recalibrate compass if you feel so. P3 rejects if it's a bad calibration at site.