This was a new one for me today. I woke up to an overnight email telling me that:

"We wanted to let you know that a recent transaction has been made from your Household Account by yourself or one of the Members of your household. This has resulted in a reduction to the Avios held in the account."

It turns out that the activity in question was for a total of 0 Avios as shown in the screenshot below. The upgrade was using a GUF and not Avios. All very confusing and unclear. I applaud the intent which is to make the household account owner aware of new activity but it would be helpful if the information provided was meaningful and accurate.