It happens to all ages of systems, new and old. It generally happens in the south but I wouldn't rule it out just due to geographical location.
Dirty sock smell happens when swapping from heat to cool. So she says it happens when aux heat is operational - My guess it is happening in defrost which will turn on the aux heat (she sees this displayed) and it shifts into cool momentarily.
When you return, force a defrost and then go sniff around. That's how I duplicate the issue when it's a complaint. I attached a PDF below.