First, the charge state (number of bars) on the display is for the Traction battery. Roughly the bars represent a charge state between 80% (all bars lit and green) to one or two purple bars as it nears 40%. The blue bars indicate the normal operating range of the traction battery. The two bottom bars are used to indicate the battery is at the low state of charge.
Normally the top two bars are only lit (and all bars green) when you are regenerative braking and fill the battery to capacity (80%). If you continue down a hill then the systems must protect the battery from over charging. To do that, the engine is run as an air compressor to burn off the excess energy (it is not burning any fuel just converting the extra energy from breaking to heat). If you have the air conditioner turned on then that may also help use some if the extra energy as it runs off the 201v traction battery).
The two lower bars will light purple when the traction battery is approaching its minimal discharge point. This can happen if you are using a lot of electrical energy such as running the A/C. The 201v traction battery supplies the voltage for the car's 12v electrical system (headlights, fans, computers, charging the 12v battery) via a DC to DC inverter. When the battery reaches to two lower bars, the engine may start to recharge the traction battery up into the "blue" level (3 bars and above lit blue).
The car may drain down the traction battery during stop and go driving at a rate determined by how much current you are using while stopped or driving in stop and go conditions. If parked in a lot with A/C running, radio on fan on etc, then the battery may go down faster. The car is "smart" and as long as you are in the "Ready" mode the engine will start to keep the charge from getting too low then shut off again.
I have experienced more of a drop in the traction battery when parked and waiting in the car. This is somewhat more evident when the small 12v battery is not fully charged or is getting weak. For me this seems to happen about every 3 years.
Hope this helps explain what is probably going on in your car. Make sure of how good your 12v battery is if it is more than two years old. Also, leaving a light on in the car for extended periods while powered off can cause complete discharge of the 12v battery and that small battery doesn't seem to recover well from that.