Voltage doubled on capacitor
Once the capacitor is charged, it's a 3V source with a polarity opposite the battery, so the net voltage is 0.  If you flip it around, it's another 3V source in series with the battery and with the same polarity, so they add to 6V.

That said, why it only discharges to 1.5V is more of a mystery.  Is the cap polarized?  An electrolytic/tantalum might leak in the reverse direction, but one doesn't often encounter these types in the sub-uF range.