Can a low 12volt battery cause the red triangle

It’s possible.
Charge it up and see if it will hold a voltage.
If not, replace it and keep the new one on a maintainer if possible

 

You had the diagnostics You cleared them away. This is why you just don't erase codes at least write them down take a picture of them on the screen something because then you're making posts well like this I guess The red triangle if you look in the owner's manual doesn't they call that a master warning light or something along those lines that generally lights up to let you know that something is amiss not necessarily what to something is those are the other lights like if you open the door while the car is in ready drive mode the master warning light comes on obviously to let you know that something major is going on yeah the door is open while you're driving. Etc etc I don't think just your master warning light being on indicates a low voltage situation it means hey something is going on with the car here's some other lights you need to look at while you're looking at this one and maybe it means you should be getting the code scanner and retrieving the codes and if your code scanner can't remember them for you write them down before you clear the code to see if the car is going to drive so that when you have discussions with whoever you at least have something to fall back on instead of I had a bunch of codes but I cleared them and I don't know what they were whoops.

 

How much under? What was the voltage specifically?

It's not likely if the battery's voltage was above 10.5 V and didn't drop below that when you tried to make the car READY.

What would be useful is knowing what codes were set by the two systems indicated by the lights on the dash. With the codes, we could offer you better advice.

 

These lights mean one or more codes were set in the system indicated by each light. That is, the VSC ECU (part of the ABS ECU) ECM (engine management computer), brake ECU, the HV (hybrid vehicle) control ECU, and possibly the battery ECU.

If or when any of these lights return you need to read the related ECU to retrieve the codes and report them here for advice on what to do. If your scanner tells you there are no codes, it means that is not capable of reading all the Prius' ECUs. Even if it does retrieve some codes, it may not retrieve all codes as some entry level scanners can only interrogate two or three of the 11-14 ECUs the Prius has. If you find yourself in this category, there is a review of capable scanners here.

 

While it won't work for the triangle light or the engine light, there is this easy method you can always use to get the brake and VSC codes without relying on a scan tool at all:

Blink (a/k/a Flash) Codes – How to. | PriusChat