How can I be sure there are no gas leaks around Gas Oven/Range Combo?
The sniffer is an instrument that is used to detect gas in the area.  Once that is determined it is always recommended to use the search bubbles for searching on the individual joints, and fittings. If you are looking for leaks around an appliance like an open cook stove, it is understood that you must calibrate the instrument to eliminate the background 'odors'. Background 'odors' surrounding this type of appliance are to be expected especially when cooking and working on the unit. If you have been away from the house for several hours and upon returning, you smell gas in the area, you know you have a problem.
You already knew you had a leak around the stove.  The protocol would be to now use the bubbles on every field connected joint to the stove. You are on the right track.  After making the repairs to the joints that made bubbles do a retest.  If all joints pass with no bubbles you can expect safe operation.  The sniffer at this point is out of its class.  Additionally, should you see any additional joints that at not generously taped or doped, take them apart and tape or dope them. (flare joints excepted) (All the joints inside the range will have been factory tested and can be expected to be tight.)