Sudden, noticeable drop in gas mileage + weird MFD behaviour

So I drive a 2014 Prius+ and commute to work everyday, about 20km each way. I'm one of types that keep a really close eye on the consumption and Energy Monitor. I know exactly what gas mileage I can expect at various locations and frequently try to best my record.

This Saturday, I went the same route as I always do to pop by the office. I recall hearing a beep at some point while driving. The kind you get when you get out of EV mode, except one single beep. I was really confused because it was so out of place. No lights flashing or warnings anywhere. I had 1 bar of fuel left and a range of 60kms. I figured that maybe the beep was about that. No flashing warnings about fuel though.

I was coming up to one of my "checkpoints" and 4.9L is typically on the high-end at that point. If I get a 4.6 or below, I have a good chance of beating my record at my destination at work, which is 4.1L. This time it was 5.6L. That was alarming to me. I had recently been in a 1000km+ roadtrip with speeds of up to 135kmh and I averaged just 5.1L on the whole trip! I thought maybe the low fuel messed up the count. I topped up but no difference.

I was growing really concerned. The traction battery seemed ok. It had good range in EV mode. As I drove back home I noticed that the Energy Monitor looked really strange. The flow reading for the electric motor and battery was really sporadic at hwy speeds. The flow indicator arrows would appear for a sec and then just blank for a few moments and appear again for a sec. This was unusual and it occurred to me that the traction battery is not assisting at hwy speeds and hence the higher fuel consumption.

I've tried searching and browsed pages of threads to no avail. Can someone recognize this problem? The car has 220000kms and I'm the second owner, having bought it a couple of months ago from a Toyota dealership. They had told me that the 12v battery was fine at the time. I checked it on the MFD and it read 12.3V on ACC.

I'd really appreciate ideas!

 

Thanks Bisco. I held off for a few days on posting. Bizarrely, things have gone back to normal since Wednesday. Saturday-Tuesday was significantly lower mpg. No change by me to anything and it's totally fine now. I have my maintenance coming up so will change 12V battery, plugs are due a bit later but might do them. It has an impeccable service history in the same Toyota dealership I bought it from so not concerned about the mileage, which is, so we're clear, less than 140K miles. Sure it's high but not a concern.