Is this reversed P-MOS next to the battery for some other purpose?
This is actually a reverse polarity protection circuit. Keep in mind that if the battery is the supply, the drain and source of the PFET should be transposed as shown in the following circuit. To turn on the PFET, the gate must be pulled low. If the voltage is reverse applied, then Vgs is not pulled low and the PFET remains off. The reason that the PFET has to be connected this way to to prevent its bypass diode conducting if the voltage is reverse applied.