TPMS indicator won't go away

I have a 2010 Prius which has been almost completely trouble free since I bought it (new). Yesterday the tire pressure indicator came on. I got my jumper/inflator box out and checked it out. The left front tire looked low but tested 35. The right front tire also tested 35. Both rear tires tested around 20.

Re-inflated the rear tires, but the indicator light is still on.

I rechecked the inflation on all tires and all are as specified on the sticker on the door. I then followed the procedure in the owner's manual to reinitialize the TPMS. The light blinks three times, and stays on. I tried it several times. I have restarted the car several times since then. While driving on the expressway this afternoon, I noticed the indicator was blinking for a while, then went back to solid on.

I think there is something wrong with the system or with a sensor. The car is almost due for 60K service and I guess I can wait until then to have the dealer look at it. Are there any other things anyone can think of to try? Are there other causes for this light than tire pressure?

Thanks for any suggestions.