I've had gas co out numerous times as the mercaptan smell is noticeable. To date they've never been able to set off their high-priced, presumable sensitive(???) detectors at any location.
I've become convinced by characteristics it's a remnant of the odorant having been left after repair/replacement when piping has been open. It's noticeably stronger in the well house if get a little water on the floor near where the heater sits which I replaced the old "wild" pilot valve on a couple of years ago with one of them newfangled, gee-whiz thermocouple-countrolled doo-jobbie ones a couple of years ago.
I've not gotten one of the n-gas wall monitors thinking the likelihood of them being sensitive enough to help if the gas company can't find it with a portable going around all the piping and end devices was likely near zero.
It is disconcerting on occasion, however, 'cuz one wonders for absolute certain whether it's just getting missed or what...