Any and all suggestions and humor welcome on this one.

Last week I was given Football tickets and left for the weekend. Upon
return, as I opened the door between my garage and basement (at the foot of
the staircase), I was struck by the horrible odor of mildew/stagnant
water/something decomposing. There is no water on the floor of the garage
which slopes up toward the entrance door.

I began searching for the source of the odor. I had set the thermostat back
to 62 and forgotton to turn of the water suply to the humidifier on the
furnace. I intermittently use an ionizer and had left it on high over the
weekend.
This is what I checked upon return:
1) Furnace & humidifier, but saw nothing awry or smelled no bad odor near
the humidifier or furnance.
2) Floor vent in laundry room, has water in it and no bad odor.
3) Bathroom in basement, nothing abnormal
4) All water pipes in basement, nothing unusual
5) Removed all garbage from house
6) Threw away a mildewed scrubby pad in kitchen sink (that did stink).
7) Checked mat at base of staircase, no odor or moisture.
8) Checked storage closet under staircase and landing, no odor or moisture.
9) Checked clothes washer in basement, nothing abormal, no odor.
10) Checked all bathrooms, toilets, sinks, no odor.

When the furnace is on, the odor is dispersed and "blown" around the house;
but when the furnace fan shuts off the odor is still at head height at the
base of the staircase in the basement. What are my options?