Why did my 2009 Prius lock me out?

As I write this, I am at home and safe.
I am just trying to troubleshoot what may have caused this issue, and how do I prevent/solve it for the future.

I went to work at about 6pm last night. I remember that when I was trying to lock my car with the key fob, it was being a little weird and did not lock right away. After moving closer, I was able to lock the door.

I left work around 12 am today, and went out to my car to leave.
I was able to get into the car. I cannot remember if the car doors unlocked as I put my hand into the handle, or if they were just left unlocked.
I pressed on the brake pedal and pressed the start button and it did not start up. I tried this about another 3 times and it still did not start up.
I inserted my keys into the slot and then tried pressing on the brake and pressing the start button. Still did not turn on.
I MADE A HUGE MISTAKE, and left my keys and my purse in the car.
I opened my door to try to go into the trunk to check if I had my jumpstart battery pack on me, and the driver door shut. It locked me out of the car, and when I tried the handle, the alarm went off.
After the alarm turned off, I tried to enter the car again. The overhead light was on and the car made the long beep it usually makes when the keys are in the car, but did not unlock the door.
I had my phone on me and tried to call roadside assistance, but state farm outsourced it and they wanted me to fill out a long form while I only had 39% left.
I was able to call my fiance and have him bring me my spare key.
From there, I was able to use the mechanical key from the spare to open the door. I then was able to sucessfully start the car and drive it home.

My main questions are what could have caused this?
In another thread, someone said it could be the key fob. I just had the keyfob replaced at Batteries Plus at the end of January. I also was able to use my normal keyfob to lock, unlock, and start the car in my driveway when I got home.
Could it be the battery? I had the 12volt (?) battery replaced by the dealership back in 2017 because it kept dying in winter. It cost me about $180 to replace, and I wasn't sure if this indicates that I need to replace it again. I also noticed that when I started the car, the battery level on the screen was blue and two bars above half way, which I assume means it was fine.

I would like to know what steps does anyone recommend for preventing this in the future? I have work later today and would like to be able to drive to and from without a problem.

Any help is greatly appreciated.

 

Everytime I go to Batteries Plus I just usually tell them to replace the battery and hand them the fob. I assume they replaced the battery as I always get my standard key fob with the hidden physical key back. So as to whether they changed the RFID, I do not know.
I do know that it would automatically unlock before when I would put my hand in the handle as it was supposed to.

Is the RFID part something that has to be replaced with the batteries everytime I change the batteries? Is there a way to get a new RFID part for the key fob if they forgot it? The second post mentions its soldered into the fob.

I will have to check on that. Since it is now daylight out when I get to work, I don't usually use the dome lights.
I am pretty good about double checking that I hit the lock button three times in a row and look for the lights to flash, so I do not think it was that, but I will still check.

My plan in the afternoon (since it is currently 6:05am and I have to get to sleep soon) is to take my jumpstart pack to the battery and see if it was drained (since mine tells me what my battery percentage is at). I am not sure if there are any other steps to try in the meantime.

 

Thought I would give an update.

It has been a few weeks now and there has been no more issues or lockouts. I ended up having to drive my car the morning that I made this post (had to help family out). There were no issues that occured that day.

There still hasn't been any issues recently either. The battery seems fine. I do not have the multimeter, but I checked the headlight brightness as recommended and it still pretty bright.

The only thing I can think of, could possibly be that the remote is defective. I know that others in the parking lot drive the same generation of prius as me, but I was not near any other priuses when I got locked out. It still baffles me how this occured, but I am now very cautious and keep my purse with keys and phone on me at all times over my shoulder, even when driving.

 

With my purse I throw both items in the same (and only) pocket. It tends to work out fine.

To those wanting an update, the issue was the 12 volt battery.
On Wednesday I came out of a doctors appt to an unlocked car. It wouldn't start no matter what I did, and I was super careful about making sure I always kept my keys and phone on me.
The car had to be jumpstarted. Roadside assistance told me to take it in and have them inspect both batteries, as the car is getting old and it could be the hybrid battery pack.
I took it into Car Max and they did a diagnostic and replaced the 12 volt. It seems to be fine, but I just picked it up today. Time will tell if it was just the 12 volt. They said they tested the hybrid battery and that everything seemed fine.

My suspicions are that time and the weather played an impact in the death of the battery. Wisconsin did go through a polar vortex this year, and my town reached an alltime low of -50 degrees with windchill during January. It is currently pretty warm around 43 degrees, so hopefully the battery will last me a while.

They put a toyota true start battery in, and I have the paperwork. It apparently has an 84 month warranty, which is good incase this happens again.