False contact.
 

Where can I find any information what is false contact and how to recognize it in order to avoid chasing it? I have checked all manuals but found nothing on the subject.
Has this been a feature from the beginning of DW, or was that introduced later in RA DWX mod?

It's in RA. You can find it in the included frequency sheet. There are three false contact signatures each for 50 Hz and 60 Hz simulating small, medium, and larger craft. There's no way to know you're chasing one until you verify the signature (or in my case in a P-3, that you get no MAD contacts :haha:)

You have info in USNI Reference in game in Civilian section:

http://i.imgur.com/sWrLvTU.jpg

http://i.imgur.com/HPGwLMC.jpg

I think it was first time in DWX mod for Sub Command but i'm not sure.

Thanks for info. BTW, using torps on false contacts results in most cases with cut wire.