These were my thoughts on it....
Re. 3 Door or 5 Door Vauxhaul Astra/Hire Car
Taken a little look at the car rental places in Dover to see whether any are any offering 3 door Vauxhall Astra's. Can't find any that list it as an option. Mostly their "compact" models tend to be Vauxhall Corsas or Fiats. Seems to suggest it's more likely to be a 5 door but of course not definitive. (Before this I did like
@Parmalade 3 door speculation as it seemed a good possible explanation for both front doors being open)
Interestingly when I went to do a search on "Yellohire.com" the location of 'Ashford (Kent)' came up automatically in Vehicle Pick-Up/Drop-Off location. Perhaps it's just caused by lots of people like me doing the same thing...
If it was this place he used. The hire car could have been picked up from Dover and dropped of at the Ashford branch. The 'Yellohire' company is also significantly cheaper than the other more well known Enterprise/Europcar/Hertz.
Other than 'Yellohire' it looks like 'Enterprise' is the only other company that has a branch in both Ashford and Dover.
Cheap Car Rental & Nationwide Car Hire only at Yellohire
All speculation and MOO