What happened to all of the ships, there are no more?

[WernherVonTrapp]

Well, if you're in high TC, sure it's going to appear as if a lot of contacts are showing up. Slow down to real time and it'll be more like hours or days that these are appearing. Like DP says, these contacts aren't going to sit around waiting for you to come to them. They're contact reports sent in by other units, subs, etc. and they only report their last known location. They don't report any change of course the convoy may take. Sometimes you can guess their destination and plot an intercept course, but if you're several hundred nm away when you get the contact report, your chances of running into it are slim unless you get very lucky.