2004 Prius thinks key fob is in ignition; kills battery

Are you having trouble with your dash not coming on when you turn the car on?

 

You may have a problem with your combination meter, where the dash lights will not come on at times. When this occurs, it's often hard to shut off the car, and it's possible the car will have certain dash lights/MFD screen still on when you step away.

 

So let me ask you this when this happens if you were to reslot the remote and then pull it out does the beeping now stop? If yes pull the under panel that contains the slot and the dimmer and remove the amplifier which is what the slot is called read the bottom label and take one from another car I have a bunch of them here and stick it in there and plug it in don't mount it to the panel and ride around the block and now try it again did that solve your problem there you go I had this on one car already I changed that amplifier slot device and the problem immediately stopped I don't know if there's an antenna in this thing or how the amplifier actually works or any of that but that fixed the problem in this particular instance which was similar to yours in the car with a black backed remote I would deslat the remote open the door it would still be telling me something at the time I didn't know what it was One day while the door was open and the car was beeping at me I just stuck the remote back in the slot and it stopped and I pulled it out and it stayed off and I closed the door I never lost a battery from this at all. And after that happened the next morning I took the slot amplifier out of a 2005 and stuck it in the car and question which I think is an '07 and that was finished

 

The problem happened after? putting in a new 12 volt ? Wondering what changed after disconnecting the old 12 volt.

Have you checked the test screens on the MFD?
Two Utube shorts that show howto get Gen 2 into the MFD test screens.
There are probably others out there too.
- YouTube

- YouTube

and a regular full tube that might help too

 

Isn't there a physical switch in the slot that detects the presence of the fob? I'm guessing hooked to the two little prongs that stick into the sides of the fob to retain it. If that switch exists and it is staying closed or open (whichever indicates fob here) when it shouldn't it would explain what the OP is seeing. Either form of failure is possible for an electrical switch. Which doesn't mean that Tombuk2 isn't right, could be two separate problems that produce the same symptoms.

Looked in the manual, on page ST-7 components related to the key slot are:

• Halfway switch:
Detects whether key is inserted and outputs signal to transponder
key ECU.
• Full switch:
Detects whether key is inserted and outputs signal to power
source control ECU.
• Transponder key amplifier and coil:
Receives ID code signal from transponder chip which is built into
key, and outputs it to transponder key ECU.
• Key interlock solenoid:
Power source control ECU operates this solenoid in accordance
with power switch mode and shift position to keep key locked in
key slot.

But I didn't find a diagram showing where either of those switches are.

Edit. Looked on Ebay for pictures of the slot, and on the back side one said Amplifier Immobilizer. So Tombuk2's answer would appear to be complete, because the switches are inside that part. This link will not be good for long...

2004-2009 Toyota Prius Ignition Key Switch Star Slot OEM 626399 | eBay