Messy Key Fob Aftermarket Alarm Situation B2784 code

Hi Everyone,

Awesome forum. Thanks for any help/advice in advance.

Bought a used 2010 Prius II earlier this month. Owner stated that something was wrong with the key fob, with CR1632 battery dying every 3-4 days. Owner bought a blank, identical key fob to get programmed and sent it to me recently after it took a while to come in.

Brought it to an individual specializing in key diagnostics + programming and he could not program the blank key fob as he couldn't communicate with car. He found the B2784 code as well as no theft deterrent code, and no theft deterrent fuse in fuse box under car hood. He stated that it could be the key eco box behind glove box (more involved) or possibly a coil in start button (less involved).

Brought it to an official Toyota service center and they tested the original key fob (the one where the fob battery dies every 3-4 days), said it works perfectly. What they did find is an aftermarket alarm in the wiring. They stated they would need to take everything (wires) apart to see where problem is coming from. $159 hourly service fee x 5 hours = $800+ tax, not including any new wires/parts that I'd need.

I could have the individual specializing in key diagnostics/programming potentially do the same for half the price, although it's not a toyota specialist and wondering if it's not worth it, in the case that an expensive re-wire is needed, or the problem isn't found and it is then re-wired by the specialist in a different way, causing problems.

Alternative would be the ridiculous (though minor) replacement of the key fob battery every few days. Would probably cause $5 a month. Toyota specialist thought the aftermarket alarm was probably paired with a self-start, a fob that neither I, nor the previous owner had. Previous owner purchased this car from a Toyota dealer, 3 years ago. I'm assuming that Toyota dealer ignored the problem. And previous owner said the key fob dying every 3-4 days situation has been going on only since this past September (3 months).

When I lock car from inside, before getting out, car beeps once, and then 3 times after I close door and walk past the hood (after parking car). Never had those noises with my previous 2010 Prius. Not sure if it's the lack of theft deterrent or the original key fob just isn't perfectly paired with the aftermarket wiring.

Any thoughts or advice for this situation? I have just the 1 key fob, so obviously if it ever completely stops working, I'd need a tow and then to have the car-rewired in order to get a new key programmed. Or keep as is with the replacement key fob battery every few days, and only cross that bridge (re-wire) when I get to it? Anyone with any similar experiences or have read/heard about anything similar?

Car has 170k miles and seems well taken care of, besides this matter (clean carfax, fairly regular service history, etc.)

Thanks everyone.