Yes, temperature sensor most likely. Here's what I would do to diagnose:
1) Get a voltmeter, and put it across red and black. Should see high 3.x volts if the battery is charged.
2) Put the voltmeter across white and red, then white and black. Should show no voltage.
3) Switch the meter to ohms (maybe the 2000 scale or so), and see if you can measure a resistance across white and black, or red and black.
4) Warm the battery pack in the sun, then see if the resistance changes. If so, it's a temperature sensor.
If still unsure, you may have to cut away the plastic shrink molding to see where the wires go.