There is a setting for the groups in a file that if enabled, tells the game how often to report a group, and also how long to wait before reporting said group. Depending upon the mod and group, some groups will zig zag every 10nm or so, or maybe 24nm, so there is that variable. There is also complete changes of course from say a southeaterly course, to a south-southwestern course. So, you might have an old report, that is completely inaccurate by the time you act on it. Imagine the coastwatcher that is watching the passage of the group out of the harbor, running to his radio, and the enemy shore patrol comes along, and the watcher can't x-mit his report for hours... or maybe its an "Ultra" situation, and the cracked code info on the group is deemed "low priority" and doesn't make it to the translator / intelligence officer for two hours or more. Again, old info to you that might be completely inaccurate by the time your receive it. This is the main reason not to chase those reports. However, if you do find them, they'll talk about your encounter for ~years~ in the officer's club...
