If not used for a long enough period, the water in a trap will evaporate to the point that the water no longer seals the bend in the trap and sewer gases are admitted past the trap and out the fixture drain. A vacation home makes it more likely that the drains will have infrequent use, and being at altitude accelerates the evaporation. Incorrectly routed drains and vents can also cause water running at one fixture to evacuate the trap of another (toilets are a big offender - the rush of water from a flush can suck other traps dry if the line is not properly vented).
But the wind seems to be key here. With a 2007 construction house, built for snow, it's probably extremely airtight. It could be that the (non-sewer) venting in your house is such that when the prevailing wind blows, it's creating negative pressure in the house, and this, in turn, is causing air to be pulled into the house through the drains (especially if you've got any dry traps).
Next time you're testing
-open some windows on the windward side to ensure the wind isn't creating a negative-pressure condition in the house (not in the room where you're getting the smell - the point is to equalize pressure in the house, not vent out the smelly rooms)
-run water through all the drains to ensure the traps are flooded
-No smell? Close the windows, wait a few minutes, then sniff around all the drains in the two bathrooms.
-No smell? Flush the toilet in the upper bath (full flush), wait a few minutes, and do the sniff test.
-No smell? Flush the other toilet (full flush), wait a few minutes, do the sniff test.
If the smell returned upon closing the windows, you've got an open vent somewhere, being exacerbated by an airtight house. If you're POSITIVE all the traps were flooded (including furnace/ac condensate lines, and washing machine drain), I'd be looking for an open vent inside a wall or attic.
If the smell returned after flushing either of the toilets, you've got a vent problem on one of the those lines. If you hear a gurgling at any nearby fixtures when you flush a toilet, that's a clear sign of poor venting.
If you get the smell with all traps flooded and a windward window open, I'd guess you've got an open vent line inside a wall.
One other obvious thing -- look under the bathroom sinks and verify that they all have a real 'P' trap (google it). No trap or an 'S' trap are both bad news (and would fit with the tight house/negative pressure scenario). It shouldn't have been built (and shouldn't have passed inspection) with anything but a P-trap; but if the apprentice did that room, and the inspector wasn't thorough, it could happen.
One other possibility (kind of a stretch). If the vent on your septic tank admits air to the tank, and it's positioned relative to the wind such that positive pressure develops near the vent (wall, slope, hedge, etc, on the leeward side), the tank could become pressurized. Combine this with a low-pressure condition in the house, and your house becomes the vent for the septic tank. This could also apply if the tank vent isn't venting. If your traps and vents in the house are working, a pressurized tank shouldn't be an issue. But any problems with your venting would be magnified by a pressurized septic tank.
