Inside and outside of the car, only colder temperatures, lets say below 55, and maybe only when tank is low on gas (a couple more attentive observations of this over the next few gas fills to confirm consistency).

Any experience of this? Mechanic didn't suspect fuel leak on shorter examination when I recently had other work done.

 

I've had fuel hoses do that. They become brittle and develop tiny cracks that you can't see. When the temperature goes down, the hose contracts and the cracks open. The cracks are so small that the leak isn't big enough to drip, the gas just evaporates as it comes out, giving you the smell.

This is difficult to find because you may not even be able to see dampness on the hose. Of course it will get worse over time. For now, try to figure out the general area and inspect all the hoses in that area.

 

This is a huge problem on my 91 300zx, and the only fix is to tighten down the fuel line hose clamps. I never looked into the maxima because I don't notice any fuel smells here. This fixed my issues with the 300zx, so that is where I would start. The only other thing would be a small pin hole leak somewhere. With that though cold or hot you would smell it. Not as bad hot as cold, but still smell it. Also try changing your oil if it hasn't been done recently. If it is old oil, it is contaminated with fuel. Not sure to what extreme, but nissans get pretty bad to where you can definitely notice it. Good luck and keep us posted.

 

Isn't there a control valve that causes this problem, do you only notice when you turn off the motor?