I catch a light whiff of gas in the trunk sometimes but can't figure out where it's coming from this time.

The same thing happened on my other XJS ... the odor was strongest around the ECU when you opened the etrunk lid ... it also came from arouond the secondary tank under battery ...

I removed the battery cover and flashing underneath and sniffing the end of the foam cusion around the fuel line, the smell was pretty intense.

I removed the secondary tank and when I undid the brass fitting where it goes through the wheel arch to the next section, the 1/4 olive washer had been seated badly and was emaciated with a deep crease on one side ... evidently someone had ( this job was more than 8 yrs ago ) dismantled this part before and tightened the brass fitting back on at an angle, partly flattening the washer.

I slid a new washer on and touche! ... it still smelled of gas

It took a while but after a few more repeats I eventually seated the washer exactly in the right position and stopped the smell.