Well, some basics from my understanding and experience... not a professional. One purpose of a trap is to prevent normal gas from the sewer from wafting up into the house; if there is nothing blocking the gas, it will... the trap should be filled with water, thus fulfilling the job. If a drain is not used, it will dry up eventually and thus allow gas to flow freely- smells bad, not safe or healthy. We had (I don't know all the technical words) a house trap that was at the point of exit from the house that allowed access to the drain line, and the cap popped off (was not set back correctly by a plumber) and a terrible odor was wafting (I like that work) throughout the house, was difficult to locate where it was coming from. I had the whole fire dept (told them it smelled like gas;-) there investigating... lots of cute rescue-type men in cute fire gear ;->... they determined it was sewer gas and firmly tapped the cap back on, and voila, smell gone. bodica has a good point, although I would think there would be some moisture leaking as well as gas if the seal is not intact. Again, the wax seal is after the toilet trap, thus gas could flow freely. Check all drains in that room- any sinks, shower/tubs, etc for either "dry" or break in the line. As far as the septic is concerned (had to learn all about them when we bought our current home with a failed septic), it should not be an issue if this is occurring in only one area- a failed septic will show itself either in the clogged leach field, backing up, slow drains, etc. The type of system you describe- pump-up- just means that the tank is likely on higher ground than the drain, and needs to be actively pumped into the tank. If that were not functioning, the problem would be similar to a failing system, with back-up on the lower floors first. Not likely what she seems to be experiencing. If the smell is detectable in ANY other part of the house, perhaps it is just collecting in that bathroom? Could it be coming up through the walls? Is the drain line located below that bathroom? that it may just be drifting to that high space? Hope this gives you some things to investigate. Try posting on the plumbing forum, they have been most helpful.

If your not using that drain you should pour about a quart of water in it monthly to insure the trap stays full. After you pour the water in you can also pour a couple teaspoons of cooking oil in the drain. The oil will form a film over the water and retard evaporation.

There is no such thing as a shallow P-trap, what your looking at is a "closet bend", which is simply a long radiu 90deg elbow.... IN Plumbing vernacular what you call a toilet is properly called a "water closet" and the round ring on the floor that the water closet is mounted on is called a "closet ring". From your description your closet ring is mounted directly on the top end of the closet bend, which is fine. If the drain line was deeper they could put a length of pipe from a closet flange to the closet bend, but in your case its not needed. Not only is there NO P-TRAP on the line, code prohibits installing a P-trap there. For a water closet the trap is built into the bowl section of the water closet and if one was to install a P-trap on the line, that would be double trapping which is code prohibited. The bottom line is,,, the answer to your last question if "YES", you are worrying for nothing.

If "gas meter thing"plumber brought is for detecting natural gas I believe you are on the right track. The utility companie's line might be leaking with gas following your servce line into dwelling. Adding propane to raise line pressure during coldest part of day is common pratice. If you feel compelled to help plumber locate the problem try this. Fold a clean cotten cloth several times and hold it against outlet of grill tank. Make certain to keep tank upright then open valve for 2 or 3 secounds. Does cloth smell similar to odor in basement?