Greetings everyone!

I have looked everywhere on the topic of sea mines mainly relating to Uboats but also other nations and vessels that set them. My main question is... well... how did they find out out when a ship sank and the grt and so fourth. I see that many many ships were sank with mines but I can never find how they knew so much about a ship when they placed mine and moved to another location. Was if aircraft? Enemy radio chatter? Other ships? Going back to the site were they were laid and looking for a wreck? Or was this all found out after war? Rather nit picky question to ask but I am burning to know!

Thanks and cheers!