Hi Oakdog! Welcome aboard!
The ghost plot that you see assumes that the contact won't change course but sometimes it does. Also the scale of the map might mean that it looks like you're supposed to meet the convoy, but in fact you're 30-40km away. In cases like that, drop to periscope depth, turn off your motors and head to the hydrophone and give a listen yourself (Use the Home/End keys to turn the 'phones). You might hear it while your sonar guy won't.
The other little ship icons that pop up represent other reports of ships in the area from other friendly ships or other uboats. Just another way to make finding targets easier for you.
Eariler I was trying to intercept a convoy and plotted an intercept, but after a while realized I'd been underwater when I started it! So I surfaced, plotted a course far in advance of them, then back up along their path. Due to fog I didn't find the convoy till I was 2km away from them, I was actually INSIDE their destroyer screen. Niiice. heh
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