Finding and Elliminating source of odor

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?
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Duct cleaning? Tom
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Look for a dead mouse in wall cavity.
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1_Patriotic_Guy wrote:

Likely a dead critter in the wall or if you have a craw space somewhere check that.
--
Joseph Meehan

Dia duit
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Check the chimney. There may be a dead squirrel inside.
I used to occasionally smell something bad in my master bedroom. One day we found that we had a high concentration of CO in the basement (my family almost died). Turned out there was a dead squirrel inside the chimney blocking the vent of the waterheater. We removed the dead thing and put a screen over the top of the chimney, and now I have not smelt the odor anymore.
Jay Chan
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clipped

into all of the sink drains, in case some dried up and allow sewer gas to back up.
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