Those might be radio reports. When you see one, pull a move waypoint over near it. Do you see a small 'shadow' version of the contact moving as you move your waypoint? If so that's where it'll be when you reach your waypoint. Simply move your waypoint over to where it meets the moving shadow of your contact and travel there.
If you don't get a visual contact once there, drop to periscope depth (press 'F' by default) and wait a bit. Your sonar guy should pick up the contact on hydrophone and you can move in for the kill.
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