The ecu knows when the heated FRONT screen is activated as it has a circuit all it's self with a message sent from switch to the ecu when screen is turned on, there is also normally a auxillary air valve, near the bulkhead usually that opens to admit extra air, even though iscv controls idle speed, HFS have the AAV still.
If the car was idling at time of test with all that on, it's fine, unless the car was originally equipped with a Calcium/calcium battery, if it was it's low ish, but if ok when held at 2000rpm it goes up around 14.5 - 15V, calcium systems run slightly higher.
tabetha