Old thread
Morning, Scott.
Hope to find you and your loved ones well.
Good hypotheses. VOC or Volatile Organic Compounds. (VOC) that smells like burning rubber can/may be often from thiophenol or other sulfur-containing compounds.
NOTE: some combustible gas detectors or gas sniffers as many refer them to be, particularly those gas detectors using Photoionization Detection (PID) technology, can detect volatile organic compounds (VOCs).
Ask your clients did they have new floor coverings instilled or did they install a new carpet.