We came home and the basement kind of smelled of bug spray and there was a light fog. We cannot find the source as we do not keep bug spray of any kind down there.

Most "bug spray" smells are aerosol propellants and petroleum solvents. Check for aerosol cans tipped over or fallen behind something.

Older cans of spray polyurethane and paint can rust through and leak aerosol and solvent, particularly in a humid environment.