I would notice if this were outside my house. It would not take constant unremitting surveillance from me to conclude that it was happening as the OP describes.
It would first intrigue me, then annoy me then concern me.
I find this very relatable with a preschool-aged DD and primary aged DD. In fact I have a similar issue this afternoon with a 2pm preschool collection around the corner from the school where DD1 finishes at 4:30. If DD2 did fall asleep (unlikely without a couple of miles’ journey to lull her off) I may consider waiting outside the school in the car, there’s a certain bliss to a bit of time carved during child-wrangling hours to legitimately catch up with work emails, listen to podcasts and glaze over to social media in peace. I’d not keep the engine on though.
But apart from the unlikelihood of her falling asleep and remaining asleep all that time, I couldn’t countenance this as a regular solution in our schedule. luckily we only have this timing situation twice a week and it’s temporary while my DM is too unwell to collect DD2 from preschool and mind her while I work until I collect both DDs from 4:30 onward. And I’m actually more likely to use the time to run errands or to go home and plonk her in front of Bluey while I get some tasks done.
The thought of sitting in the car outside school like this day after day for 3.5 hours just feels like a huge waste of time and money. While I conduct a lot of my business from my phone, I couldn’t fill 3.5 hours productively, although I recognise that some could. Podcasts and dicking about online would get a bit dull every single day. I think that preschoolers who consistently have a daily 3.5 hour nap are in the minority and hours of snacks and screen time in the back of a car aren’t the most joyful way to fill their time with such daily regularity. So it may indicate that this is the only option she can bear and the alternative is really awful.
I think on balance I’d bring it up with the school in case she is using is as an escape from an awful alternative, and hopefully in the event she isn’t, she might take it as a nudge to stop wasting fuel and polluting the air outside a school and go and do something else