I am a newbie on this forum but have known Carol via other forums for a number of years.
The major problem is that no one knows where Carol was heading, therefor to say that the gendarmes need to look for skid marks is asking the impossible.
The only hope is that a walker or a hunter comes across the car or indeed Carol.
Perhaps more concentration on forums/ websites/newsmedia and facebook pages in the department where Carol lived.
If someone is out walking/hunting they will just assume, if they see a car parked, that it is a fellow walker/ hunter.
The message needs to go out on the ground,flyers distributed in supermarket car parks, petrol stations,tourist information offices, chasse offices.
Perhaps the granddaughters were ill advised not to come to the area. They could have occupied their time by distributing flyers ,because I doubt if the gendarmes are doing that.
Perhaps Swissie? or someone else, being a French speaker, can search the internet for relevant French forums.
Here is one to get started.
www.chassepassion.net/le-forum