The TF markers on the map will show if you have any sightings, radio contacts, soundings, etc. Basically the detection level is high enough for you to "know" that there is an enemy TF in that hex.
Placing the mouse cursor over the TF marker will sometimes show you some detail (not always accurate) as far as composition if you have any naval sighting of the TF.
I started a head to head game and formed a SCTF at Shortlands with all available ships. The detection level of that TF shows up as 5/2 (which means it will show as an icon for the allied player). If I had to guess I would say that coastwatchers are catching the TF forming and triggering the detection level. The detection of other TFs might be triggered by other events. The Japanese always goes first so, depending on which AI scripts run (or what a human opponent does) it is likely that you might see these results. If you are saying that you are playing both sides and the TFs do not actually exist then that is another problem entirely.
Hopefully this helps but based on what you described that is all I can think of.
Even though it is first turn for you, the war has been going on for some time. There should be some general intelligence about enemy forces in the area which might be represented by what you see on map.