Sewer Gas Odor from the Toilet

I have an interesting problem that I have not been able to resolve. We have sewer gas that seems to be coming from the water itself from the toilet in our half bath. Whenever a male urinates in the toilet, the splashing effect seems to emit sewer gas. We have no odor at any other time. Anyone have any thoughts on what we should look into?

Further details about the situation:

The half bath and the toilet room in the master bath are side by side and both back up to the garage. There is a bathroom above with the drain line coming down the wall between the half bath and the garage (behind the half bath toilet). The drain line then heads into the slab where it crosses the house and ties into the county sewer on the opposite corner of the home. We have no odors from the other toilets nor from any of the other sinks, showers, etc.

If the wax seal was gone/damaged, I would expect to have sewer gas odor more often than whenever a standing male goes to the bathroom. The toilet is a low flow and does not completely evacuate all the time (especially when the kids use it) Is it possible that the toilet is not flushing completely and leaving waste water in the bowl?

Thanks for any input.

You didn't give any details about the toilet, but I would venture a guess it is a cheap builder grade fixture from HD or similar discount house. The best toilets in the world, Toto, are made in your backyard, might be time to step up. All low flow toilets are not equal, in fact even many of the old famous name brands are garbage.