Brake Sensor problems of the 2012 Toyota Prius

I own a 2012 Prius with the technology package, including radar adaptive cruise control. I believe these same radar sensors are also used to detect imminent collisions in front of the vehicle, even when cruise control mode is not engaged. About a month ago, while driving north on interstate 5 in downtown seattle at approximately 60 miles per hour, and without cruise control engaged, my Prius went into "imminent collision mode" despite the fact that there was nothing in front of me -- no vehicles, no debris, no obstructions. Before slamming on the brakes, my seat belt was "tightened" and the brakes were automatically applied at full force, even without my foot touching the brake. This was incredibly jarring and disturbing, and could have been a serious accident if there was a vehicle behind me. Fortunately, there was no vehicle behind me at this time. After about two seconds of full braking, the vehicle "disengaged" collision mode and allowed me proceed and return to speed. I called the Toyota of seattle dealership and spoke to their service department. They were unaware of this issue, and suggested that I bring the vehicle in for servicing. They confirmed there were no outstanding recalls on the vehicle. They told me it would cost at least $100 for a basic diagnostic, and if sensors had to be replaced, it could cost hundreds or even thousands of dollars. Because I am currently unemployed and looking for work, I did not bring my vehicle into the service center. I feel this is Toyota's obligation to fix, and shouldn't require me spending money out of pocket. I would like some kind of response from Toyota on this issue. I have avoided using the radar cruise control since this moment, but the fact that it occurred while not in this mode makes me feel that the car could "take control" at any moment and cause an accident. Thanks, ryan.