Had a similar experience my first patrol in the SH4 u-boat mission. Having done two campaigns for the USN I left port in my Type IX and saw on the map a contact for merchant heading in my direction colored blue, the color of every allied ship I had seen to that point. So I submerged, lined up for the attack, when my scope came up saw a big modern passenger liner in my sights. Just as I push the button to launch my torpedo one I see the the Japanese flag on top!
Oops!
Please miss, please miss, please miss, dang!
Don't sink, don't sink, don't sink, dang!
Lets sneak away and hope nobody notices.
At least I was not arrested when I returned to port.