Car sat for a week while on vacation.On the third day back, car has message on dash display.Before I take to my mechanic I would like to know what to be ready for.What will be the most probable, short, bad battery,new alternator.?

New alternator is most likely in your future

This also happened to me recently and it wound up being my rear junction box. They needed to reprogram it. Spent $2k chasing that problem changing tons of stuff before getting to the root of the cause.

But its more than likely your alternator