Shaking/shuddering Prius after standing still for 5 days

First off, I think it is solved as I haven't seen/felt it since. Certainly not nearly as violent. But I still wanted to mention it to see if someone had some insightful comments.

After having been off to Disney Paris for 5 days with a Hyundai 7 seater with 6 people, I returned to the Prius, put the key in and pressed the "START" button. It came to life violently: it was shaking on its wheels. It became less violent after a few seconds, but I still turned it off quite fast.

Two details probably worth mentioning:
(1) I had some dashboard parts lying loose in the car, as I had put the aftermarket JVC radio from the Prius into the 7 seater and was in the process of putting it back and needed it to power ON to make sure I connected everything correctly before putting all dashboard parts back. I am fairly certain it would not have seemed this violent (but still very much noticeable) if it had all been attached properly.

(2) the car was on parent-in-laws driveway with the nose slightly higher than the boot due to a small incline.

I measured the oil after turning it off and even at the incline it was still above the minimum, but I added half a liter just to make sure.
I have left the car untouched for almost 10 weeks some years back and it didn't exhibit anything like this back then.


Ideas welcomed.

 

I can't see how water got into the engine-bay, but misfire does sound like a reason for it to make such noise.

And I can't rule out that I turned it on while taking the radio out. So the 'not-warming-up' could also be true. Would this possibly cause similar efects? How?

 

Just to mention it did it again after standing still for 2 weeks (with some torrential rainstorms during that time...).

Not so scared anymore, since I know by now it's just a few seconds. And then a few seconds of hearing the rust being scraped off the discs (we have 4 in Europe ) by the pads.

Long version of the story: we did several >1000km one-way-trips with the Prius instead of the V70 Volvo (way more trunkspace, but odly enough less legroom in the back) because of the aircon evaporater AND the condensor were broken. This was determined by a specialised AC company, (sniffer went berserk inside the car).
This included our family of 5 going to Savona (>1000km) for a cruise and subsequent little trip through Italy in blistering >40C (110F) heat. Also staying at a hotel without AC: we've had a guy looking like an ax murder walk into our locked motelroom in the middle of the night in Canada, but this night in Italy was easily the worst night in a hotel of our lives... (Guy drove an old Firebird, hair down to his butt and all around nice guy. Motel run by a couple. Wife had rented us the room and given us the key, ax-murderer-guy came in late and found husband behind reception, husband didn't know our room was taken and assumed key just got misplaced so gave him the spare key. He turned out to be as starteled as us, finding people in his bed )

I don't like dealers, so do most my work on the cars myself. Condensor is fairly easy. So I replaced it by a poorly made version that didn't fit properly and replaced it again for one that DID fit (but company refused to refund the first because it had been mounted). 100$ down the drain. Evaporator is at one and a half day's work at a stealer (so probably >1500$ on a 4000$ car? ) so I assumed it would take me 3-4 days with no way to do the complete dismantling of the dash (including steeringwheel and airbags) in a garage/with a roof over my head. So I had been procrastinating this for years and years.

Electronic failure of the CEM (Central Electronics Module) of the Volvo, causes the car to go into limp mode (including no dash/speedometer, horrible clunky shifting of automatic gearbox, etc.) when temps are over 30C (75F). Repaired myself, but repair didn't last. Found a pro, he replaced the flash memory and car works beautifully again. Discussed the evaporator procrastinating and he said he'd never heard of an evaporator dying on this car so adviced to have it retested. Found local AC company that works for a lot of dealers to do a test if evaporator was actually broken. Turns out they had Volvo posters hanging around the workshop and found no leak. Refunded the 35$ test charge if I had them fill the AC (120$, Volvo needs 1000grams of coolant, I think lot of other cars only use half...). Best guess is that the first time the condensor was leaking so much that the 'sniffing'- chemical escaping was sucked up inside the engine compartment and blown into the car's cabin and thus picked up by the sniffer. Very unlikely that a specialist would make this mistake and not consider this, but the most likely scenario. So I drove without AC for years for no real reason. Cars with AC don't have their CEM's failing as often as it gets cooled below 30C by the AC (BTW, Canadians occasionally get this error in the winter when the heat is turned on to max inside the car).

Prius struggles with the 5 of us more and more, I need the lift kit for the rear . Kids still growing, so total weight keeps going up...
And this time we needed the bench for the dog (instead of her lying in the passenger footwell for small trips), the top box and a bunch of food (gluten free kid inside). So really glad the Volvo had its AC fixed. And the reason for buying an AWD was because our previous car (Ford Mondeo with a broken AC, fixed for 600$, broke again few month later, supposedly a different part, we stopped dumping money into it then and there) got stuck on the Danish beach before I realised all other cars on there were Audi Quattro's and Subaru's. So 18 years later I could finally use the AWD on Danish beaches... Too bad they were all so smooth that even large 2wd campers drove on there, but I carefully found some pieces of beach where the AWD had to intervene
Volvo also has automatic leveling. Mechanical, but still works fine.

So that's why the Prius was down for two weeks.

 

It runs fine after a few seconds, so not really worried. But do you think I should? It does use some oil, but I thinks that's like 1 liter per 3000km (roughly one quart per 1800 miles?). Not ideal but no reason to pull the entire engine just yet imho, but if this is linked, I might try to do the leakback test.