Was your mother in law known for keeping a tank full of gas?Many elderly are known to do this.It would widen the search area considerably.If she kept the car in good shape with oil changes and such,it does not seem like a breakdown occurred.Maybe a flat tire that would be hard to repair for them.Am praying as hard as I can.Wish I could do more.
My m-i-l wasn’t driving and wasn’t her car. Never saw it but imagine car ok. Who knows on gas....
Just nowhere we’ve looked so far.