Anti theft light is flashing and draining my battery

Disconnect the battery and clamp the two cables together for about 15mins to drain the memory off the computer then try again.

Security system ( anti-theft System ) ,if this was the cause of the no start you wouldn't hear any clicks . Anti-theft system disables the starter . When does the light flash ? All the time , flash real fast when trying to start the vehicle ?
https://www.motorcraftservice.com/pdf/pats_job_aid.pdf

Some anti-theft systems need to be reset after the battery has died and/or been replaced. Check your driver's handbook for more information on this.

When my anti theft went off because I changed a battery, mind you it was an oldsmobile I had to take it to a dealership so that they could reprogram the theft system

check your fuses for it

fault on a live connection,but check all your earths aswell

Thanks Ed

Seldom people share real life fixes for the whole
world to read

it does sound like your battery is the problem but have a local shop test it and the charging system and the anti theft system connections maybe the alarm is draining the battery with a poor connection..

Some months ago I began starting the engine of my 2001 Chrysler 300M but turned it of before it really started "was sloppy with the key". The anti-theft system went on for some unknown reason. I disarmed it by disconnecting one of the battery connections under the hood with a wrench. I connected it again, the anti-theft system was off but the battery was dead. Battery was OK after recharging, I have had no trouble since then.

As far as I know, both my original 2005 and my current 2007 flash that light when the car is off. I believe that is the whole point - to show any would-be thief that the car may be "protected." A small battery can flash an LED light for weeks or months or even longer, so I wouldn't worry about the blinking LED draining your Prius' 12V battery.