The stowaway was on the outside. Therefore could have attached after they checked. I doubt very much that it’s practical to require constant vigilance from all travellers at all times (do travellers need to hire temporary guards for their vehicles when going to the toilet in order to comply with this law?)

Also, ignoring that, it’s braindead to not make an exception when the people in question self-report and fix the issue, it’s directly undermining the goals to punish people for vigilance (even belated vigilance).

But ignoring all of that, the law (or implementation) is flawed. The stated goal of the law is to discourage negligence, but negligence needs to be measured against a fair yardstick like “could a reasonable person catch this easily”, not just “were you smarter than whoever tried to hide on your vehicle?”. Defining negligence competitively like they seem to be doing isn’t reasonable and I hope these people win and force the law to be interpreted more judiciously. Next they’ll be fining old ladies who get scammed for “negligently supporting criminals financially”.