Pay attention to description... Most of the time it points you to some object that should be in the vicinity of derelict, but it's not always the case. Sometimes it says something vague and then you just have to be lucky. Derelict show up normally when using active scan burst (or whatever it's called), do it mostly near some objects (planets, rings), because most of the time they are there. Don't forget to check out gates because often they have debris or derelict fields around them.
Also, if recovered logs say something about derelicts, most of the time it's worth checking out (intel screen -> fleet logs).