It's more than one file. Each RGG (Random Generated Group) in each of the campaign (and SingleMission, etc) mis files has a setting for a percentage of a chance to send out a "Contact Report", and how often to try for a chance.
ReportPosMin=7049 - the time in minutes. a negative number means "Don't do it"
ReportPosProbability=7 - the percentage chance of the report, if the above is NOT negative.
This is done in TMO to simulate that the Ultra system was not completely set-up yet. The main bugaboo being that they didn't trust US submariners to keep a secret
. When walking the boat, look for the traffic lanes map, and use that to find a place to "lay in wait" for "unsuspecting enemies" to come by...