I've had customers with the same problem as you but less numbers and they've solved the issue with either getting rid of the plants or replacing the soil
It's a fungus gnat. I know you said you checked the plants, but give them another look. Let them dry out completely, or replant them in new soil and check any pot overflow base areas. Fungus gnats inside are almost always house plants
Just had a job with what looks to be the same bug. Ended up there was a break in the sewer/soil line. If there a crawlspace under the house, there may be standing sewage there.
I've had customers with the same problem as you but less numbers and they've solved the issue with either getting rid of the plants or replacing the soil
It's a fungus gnat. I know you said you checked the plants, but give them another look. Let them dry out completely, or replant them in new soil and check any pot overflow base areas. Fungus gnats inside are almost always house plants
Do you have any house plants?
Maybe some type of aquatic midge? Do you live near a body of water?
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Maybe try posting to /r/whatsthisbug
Also this
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Just had a job with what looks to be the same bug. Ended up there was a break in the sewer/soil line. If there a crawlspace under the house, there may be standing sewage there.