For anyone who is curious, I collected the bag on the way back through MAD. I never received any update by email or text from IB/delivery company that the bag had arrived at MAD. I could only see thanks to my AirTag that it was in MAD T4. Once I landed, I went to the baggage desk in the baggage claim area of T4. The agent took my PIR number and asked me when I had been notified that my bag had arrived; I told him I was never notified and he seemed a bit puzzled. He spoke with a colleague for a couple of minutes who also seemed unsure as to the bag's whereabouts until I showed them the AirTag location. I believe they then surmised the bag was in a locked room at the end of the hall, so he took the key and accompanied me to the room and I retrieved the suitcase. It was a happy ending, but I am thankful I invested in an AirTag because I would not otherwise have known the bag had arrived, which makes me wonder whether a good proportion of bags declared "lost" might not really be lost--their owners might simply never have been notified...