Ghost Ship?

[GJO]

Apologies if this has been raised before - I am very new to all this.
I am running SH3 with GWX Gold. Whilst returning from my third patrol (first patrol after the outbreak of hostilities) en-route to my base at Kiel via the 'Kattegat' I could see a passenger/cargo ship that was lit up like a Christmas Tree. This was just as I was about to navigate the 'Storebælt' straits - Danish waters but an international seaway before Denmark's capitulation - so a slim possibility that it could have been an enemy vessel en-route from Denmark to the UK.

The odd thing was that although I could see the ship quite clearly from the deck, with both periscopes and with the UZO (and I manoeuvred to within 500 Metres of it but couldn't make out the flag due to the glare), the crew could not see it. The Watch Officer reported 'no visible contact' and, although it was obviously a neutral vessel, I could not 'lock on' to it and it did not appear on my chart.

To use modern parlance, what was that all about?