How to track down a small gas leak

Here's a link to a leak detector, mine has been discontinued:
amzn.to/2Nxs2ll
Klein Leak Detector: amzn.to/3rAsL8r
My step son bought a flip. We noticed a "faint" gas smell coming from the laundry room area. I used a TIF 8800A to figure out where it was. We called warranty company to fix. It was "only" $61.00 to fix under warranty. Should not have been covered due to bad workmanship when it was initially installed.
I will also say that the tradesman that left the leak detector used in this video was contacted twice to return and never return our messages.