We had a quite unpleasant and mysterious problem with the Prius’ alarm sounding (lights and horn) when we parked in a quiet neighborhood front of some friends’ house. (For a square dance.) We got stuff out of the trunk, but when I locked it, the alarm went off. I opened the trunk and it stopped. Closed and locked and it started again. Opened it and it stopped. Lauren had gone into the house by this time. I snuck around the car and used a touch pad on a door handle to lock it – and the alarm stayed off!
I went in the house and verified that the panic button on my remote clicked and lit the light. I went out a few minutes later to check it out, but it worked exactly as it should. I could lock and unlock, and by press and hold, start the alarm. Another press turned it off. Uncomfortable, I went back in to dance.
No alarm when we got back in the car later, but when we got out in the garage at home, it went off again. Lauren went in the house, and I played a little more. I couldn’t get it to sound again. I googled and read various posts about false alarms on Prius, but nothing really clicked.
At some point later it dawned on me that while I perceived myself as the driver, and my fob as controlling the car, the car didn’t care whether it was my fob or the one buried in Lauren’s purse. We took hers out of her purse and the alarm never sounded again.
The fix
I’m quite confident that something got wedged against the panic button of her fob and caused the problems, though I can’t prove it. She’s carried Prius fobs buried in her purse for many years, and this is the first time there’s been a problem, but I wanted to make sure it would be the last.
A Shapelock case with some kind of dome over at least the panic button should do. I started out making raised forms over the buttons, cut out of thick cardboard (poor choice), hot melt glued over the buttons. I cut a piece of paper to what felt like about the right 
size for the sheet of plastic I planned to form around the fob. That would provide a visual size guide later.
I merge leftover scraps of SL and press it into thin sheets so it will remelt quickly. Not that I’m going to pull out a caliper as I’m forming it, but it looks like around 0.050″ thickness flexes nicely but is still sturdy. I rolled out a blob 
of SL to about the right thickness with a thick tumbler, and cut it to about 
the right size rectangle with a kitchen scissors while looking at the paper template.
It was so thin it cooled substantially while I was cutting it, so I dunked it back in the hot water. It folded over and stuck to itself, and I had to start over. Not a big 
deal. In addition to the large rectangle I planned to wrap around the fob, I cut another smaller rectangle to go over the panic button so its cover would be thicker and stiffer. A try or 2 later, I had a Shapelock wrapped fob. I didn’t want to run cold water over the fob, so I stuck it in the freezer for a minute.
No cigar
Nice try. One of the cardboard forms came out nicely, but the SL stuck thoroughly to the other. Don’t use cardboard for this. The raised part over the panic button (after I clawed the cardboard out) was perfect: a smooth dome definitely strong enough to do 
the job. The cross section was visibly thicker than the rest. Unfortunately, the case didn’t wrap around enough, and didn’t stay on as I wanted.
I cut the case up and remelted it for another try. (That’s one of the greatest things about Shapelock!) This time, I used polyethylene covers for the buttons. Instead of fighting with two pieces of soft plastic to make a thicker part over the buttons, I just cut a small piece to cover them and pressed it in place. It cooled off pretty much while I was wrassling the larger piece to wrap around the case, but I think they still welded together OK.
This one was much better. Although the poly covers came out easily, the SL stuck to the fob way more than it had the first time. (Oil washed off?) By the time I’d pried it off, the SL case was kind of sprung, and didn’t stay on well. I trimmed it up (the vision of wrapping a neatly trimmed rectangle of soft plastic around the fob for a nearly manufactured looking case remains 
elusive) and dipped the most sprung part back in the water for only a few seconds. That let me reform it just as I wanted (if uglier). When it cooled, it was again seriously stuck to the fob. Huh?
Anyway, it came out great. It’s a little bulkier that I hoped, but OK. The protective domes over the buttons look about perfect, and the case snaps on and off satisfyingly. And I’ll bet we don’t have any more mystery alarm events.