Red Triangle on 2006 Prius, no ODB code, 11.5 aux battery

The red triangle of death (shown below) has come on a couple times. Generic ODB2 reader gives no codes (other than P0446 and P0456 but it's been giving those for a long time, EVAP issues). Clearing the codes turns off the triangle. Some of the times the yellow brake warning came on the dashboard at the same time (not shown). Sometimes a black bar covers the main display with a red car icon and exclamation on the left (shown). It almost looks as if it's holding space for a message but it's blank. Possibly other squirrely symptoms I might associate with a computer browning out, just guessing.



The diagnostic display (Info + 3x headlights) says 11.5V when the engine is off. It says 11.1V while the engine is cranking. It reads 13.8V after the car is started and consistently while driving it around. I put a multimeter on the power tab by the fuses and read 11.6V while the car was off.

Mileage is okay, 42mpg as it's always been. Twice I noticed that when the triangle came on I was accelerating, and there may have been a slight loss of power but not much. Other than that, acceleration feels normal and the car runs fine. High-voltage battery symbol on the regular display charges and discharges normally. One of the times the triangle came on this battery symbol showed empty (no bars) but it promptly charged up again as I drove.

Coolant was low in both reservoirs, but I topped them off and the red triangle happened at least twice since then. Oil is fine.

Is it time for a new aux battery? Last replaced about 7 years ago, also in the dead of winter. This post says a voltage below 11.0V means it should be replaced.

Using a generic ODBII interface and the Car Scanner app (not pro).