Idle EV info

https://www.renaultgroup.com/en/news-on-air/news/how-to-prepare-your-electric-car-to-be-parked-for-an-extended-period-of-time/#:~:text=In%20short%2C%20there's%20no%20need,radiator%20hoses%20can%20become%20damaged

This type of info might answer your question. 

As far as I recall, my handbook contains a warning that the EV battery will drain in cold weather as the heaters will turn on to keep said battery at it's working temperature. I can't tell you any more than that. 

I guess your neighbour's car was stood parked during cold temperatures.

Yes indeed it was a couple of weeks ago.

These days cars have such complex electronics an ICE car can also drain its battery in 3 weeks. As for the battery conditioning - it may be possible to turn that function off when the car is not in use. It is on my PHEV - we can select from the app when to pre-condition the battery. My previous Volvo PHEV had similar settings. 

I should have added that we leave our EV unplugged at home when we go off in our motorhome, in six years plus we've had no problems. We'll probably take on board manufacturers suggestions about extreme hot weather in future but even with that battery loss is stated as minimal. 

ICE cars certainly can. I had a 2016 Subaru Forester with all singing/dancing electronics. That had a flat battery after being parked for 3 weeks. The answer was to disconnect the battery which caused other issues. Cars aren’t as clever as we're led to believe.

In the EV world, traction battery (as opposed to 12v battery) discharge when parked and locked is known as vampire or phantom drain. Most EVs do not exhibit it. My two (Polestar 2 and Fiat 500E)  have been left for a week over winter without a single percent being lost. Very cold prolonged weather should not drain a battery unless there is some mechanism for artificially maintaining its temperature. 

There is one brand that _DOES_ regularly experience phantom drain and its well documented - that being Tesla. Except it's not really phantom in that it's documented and generally well understood, and that is the drain if he car is left with "sentry mode" enabled. 

Sentry mode keeps the cars external cameras active and it "looks" for activity taking place close by the car. This can then trigger both the car to turn on it lights and activate the alarm as a warning to someone getting too close. It will also record the events on the cameras for later download, allowing the owner and authorities to act the car is damaged or broken into. 

But the use of that relatively powerful computer will drain the battery if it is not left on charge. 

Viatorem - is your friends car a Tesla, and do they use Sentry mode? If so - perhaps problem solved?

I always find it strange when I hear a locally owned Tesla charging up on the street, it's very noisy (there is an explanation.) There are a lot of variances in the design and development of EVs and it's true to say that individual manufacturers will give details just like my example of Renault have done, so that's the place to look if you want answers, leaving anything "on" will obviously make a difference.

Its a Jag, I don't know if it has a sentry mode. I know that all batteries have a self discharge rate but my understanding is that its around 3% per month for lithium, my guess is that the car may either have a fault or something juicy was running in "standby mode"

A websearch reveals storage procedures for battery cars, quite different advice from some manufacturers. https://www.autovolt-magazine.com/how-to-park-your-ev-for-long-periods-manufacturer-guidance

https://www.holidayextras.com/airport-electric-car-charging.html#:~:text=To%20charge%20your%20electric%20car,have%20electric%20car%20charging%20stations 

I was intrigued by your quote of £50 (£100 in your OP) for airport charging and wondered where this came from? I think these prices are perhaps more realistic.