And yes he has a placard but he forgot to put it out on a day when it was raining hard and he was in a hurry to get out. I remember years ago he also got a citation (in person) when he dropped my mom off somewhere and didn't notice these really bizarrely located lines that were faded, along with noncompliant signage. But an attempt to appeal that on the basis of bad lines and signage didn't go anywhere, and it was 400 miles from home so it was impractical to go to court to argue his case. They just paid the bail and moved on with their life.
But it got really weird because this time because he couldn't find the ticket. It was placed under the windshield wiper but that's buried low on his Tesla Model 3. Then he turned on the wipers and they say something flying off as they were moving and couldn't figure out what it was. My dad then came to the conclusion that it might have been because he forgot to display the placard, but mom thought it might have been some ad placed on cars.
He went immediately to the police station to ask about it but they had no record of it. Today my dad asked me to go with him during lunch to see if maybe he was cited, since I'm generally better at explaining this kind of stuff. So I did. It was bizarre though since the communications are through a phone handset and there's a security window. But I was able to find out that he was indeed cited, but I mentioned how he turned on the wipers and saw something fly off where he couldn't track it down. We waited a while for them to perhaps obtain a copy of the citation but all they could provide was the citation number on a form.
Apparently they have an appeal process, but the appeal has to be sent to their citation processor at a PO box in Denver. They were kind enough at the police station to make a color copy of my dad's disabled parking placard to send in. I included a copy of the form with the citation number and also a copy of my dad's disabled parking receipt. Not sure what the next step is, but this should be pretty easy to resolve. Just not sure if they're going to charge some nebulous fee for their time.
The city of San Diego was the only place I could find with a published process for something like this. They apparently charge a $10 fee for any kind of fix it citation. However, they seems to ask for a copy of the registration too that they'll match to the receipt.