vehicle immobilizer/Theft deterrent indicator light came on after 10 yrs now batteries are drained

My 2005 prius with 81k on it just had the Hybrid vehicle immobilizer / Theft deterrent system indicator light spontaneously come on during a road trip where she was heavily loaded for the very first time since I have owned her for over 10 years. I stopped at a rest area and when I got back in it was on...immediately my batteries started draining. They are in 50wh mode which has never happened before, and charge will driving, but if I shut down for even a few minutes they are drained again. The dealer told me this was nothing to worry about and that as soon as they fully charged this would stop. Has anyone experienced this issue and/or do they have a remedy or fix. Any input is appreciated. Thank you.