What is Prius trying to tell me with all this beeping?

FWIW, based on the symptoms you reported it sounds like the Prius "thinks" it is NOT turned off.

All three of the items you describe WILL happen if the Prius is still on when you get out.

 

It seems unlikely with a 2015, but always check the 12v battery when weird things start happening.

 

don't recall anyone reporting this before. any aftermarket equipment?

 

After checking things in "accessory" mode, "OUR" Prius will do "ALL THREE THINGS" that is described above.

Based on what is written in both of your posts, it "sounds" like you might be having a problem with either the power switch OR a ECU in the circuit. Either way, since the problem is sporadic it will be difficult to troubleshoot and repair.

At this point it really is best guess as to what the proper fix would be. You could throw parts at it all day long and never fix the problem. I too question if there might be some sort of aftermarket device installed on the car.

Bottom line is this. It is NOT normal for the Prius to enter "Accessory" mode (orange LED on power button) by pushing the power button ONCE when shutting down. Normally to enter "Accessory" mode one has to begin with the Prius being completely "OFF", then pressing the power button ONCE WITHOUT pressing on the brake pedal. Also the 12 volt outlet should be powered down when the Prius is OFF, however it will be hot when the accessory mode is in use.

 

It is possible that you have a faulty power button that is causing the problem (acting like you've hit the button twice causing a power off and then enter accessory mode).

But in case an ECU is causing the issue, try disconnecting the 12V for a minute or two in order to force a reboot and see if that clears the issue.

 

I think I also suspect the power button. I would think that it would be made to not "bounce." If that's not working, a switch can see one press as several because when you press a momentary contact button, the contacts can sort of chatter back and forth for a tiny fraction of a second.

 

Sounds like a good plan, Johnny.

 

Have you read this? Might be related:

ice randomly starting and stopping | PriusChat

 

My 2012 has been perfect until I wired a dash camera into the accessory power circuit. I've notice 2 things. 1- sometimes the camera does not shut off or come on when the car is started or shut down. 2- I have gotten the beeps as described above. Last night I shut down and waited for the camera to beep and shut down...it did not. Adapter was showing power. Car was off. Power button unlit. Got out of car....beeps.... got back in, went to aux then shut it off. Camera shut down.
Today... had same thing happen at shutdown. unplugged camera. Camera shut off. Plugged in camera....did not start back up. Aux power now off.
It's behaving like the power circuit is being held on by the camera.
Have cycled the power a dozen times without the camera plugged it.... shuts down each time and no beeps when exiting.
Have the car pulled in and the meters out.... probing time!

 

Update... meter says everything is "ok". Voltages are what they should be when on and off.

That said.... I did find something.

I was suspicious of the power button... so over a period of a week I made sure I was very deliberate in pressing the button. And I used what I would describe as a "long press". Even with the dash camera connected I did not have a single issue. I can not know if the button is needing replacing but maybe I'll price it out and see if it makes a difference. Meanwhile, I just "casually" press the power button.

 

Have not changed the power button....but... I did swap the Pilot camera for a Garmin DashCam 55 and I have not had the problem. So... yesterday being a nice day... I sat in the car and swapped cameras back and forth. (Also borrowed 2 other cameras, another Pilot and the other just said Star on it. Owner says he paid $40.)

Guess what? All of the cameras except the Garmin produced the problem.

I'm going to go out on a limb and guess that the cheap cameras do not have a blocking diode in the power circuit and Garmin does. With the more deliberate press of the button it might be that we "step over" the glitch time frame. Regardless, I am not running the Garmin and not looking back. (BTW...The Dashcam 55 is really cool. In addition to full HD, you can talk to it. About a dozen times now I've told it to save a video segment because of something that happened and it has don't so. You can also tell it to take a picture which was really cool when we went on vacation... I had it take a picture of various milestones along the route and ended up with a photo album of the trip when I got home. (ok...enough unsolicited endorsement ..))

 

I think you have hit the nail on the head with your theory about feedback from your accessory device. I think the battery in your device/ devices is the culprit. There are products that are designed to plug into your power outlet that keep the ECM alive when replacing your 12v battery. This is accomplished with a 9v battery and a pigtail that plugs into the power port. This might also be a sign that your 12v battery may be dropping below the designed voltage limits that are required to keep the ECM alive.