Don't listen to AP's nonsense - the AP-worker-signing-on-your-behalf arrangement has been in place for almost two years and it does not change the fact that the AP worker has left the parcel without doing what they were supposed to do.

If you're certain that nobody called out, then we can only conclude that AP have checked with the worker who left the parcel, and the worker has said that someone yelled out to save their own neck. They know very well that they're not supposed to just leave a parcel requiring a signature without sighting someone at the address.

My next question to AP would be - what is the name of the signee whom they supposedly sighted at your address, and whom they signed on behalf of? If they say you, then you can obviously assure them that this is false. Keep at them, and as someone else said, mention the ombudsman if necessary.

Overall, I think AP do a good job but this is definitely one area that needs improvement because this seems to happen often. I have parcel + signature as one of my postage options and I have had buyers ask me to refund the fee because this has happened to them, and they're upset about having paid for something that was not used. I've then had to go about proving that I did pay for signature for their parcel, and the AP worker just did not bother ringing their doorbell or whatever. Good times.