2003 chevrolet impala

Smell gas especially in rear of car. Mechanic can't find problem. He has done smoke test.

when do you smell gas? did the mechanic that checked your can notice the smell? Try to find out under what circumstances the gas smell occurs.

after a fill up?
when it's close to empty?
After a short drive and you have come to a stop?
only after you turn the engine off?
only when the car is running?

looking for a occurrence that can be traced...

It's weird the Mechanic can't find the problem..

I had the same problem, but the mechanic did find the problem. Apparently it was a defective fuel pressure valve that is prone to leakage. He replaced it, but after a day, it happened again. I brought it back and he replaced it free of charge and said that is a known problem with the OEM part. The replacement did its job for about a year and then that one went as well. Try replacing it with one that is NOT an OEM replacement part.