Let's all read that passage as it's vitally important that we're all on the same page.
Regency Sourcebook, page 79,
Penetration entry, second paragraph:
Penetration is accompanied by gravitic "indentation" and "ripples" which can be detected by sensors with a hard fire control lock on the departing ship. Proper analysis of the penetration angle, jump envelope configuration, and entry vector can allow a prediction of the likely direction and distance of the jump. Prevention of such lock-ons is one of the main missions of rear-guard screening forces in space combat. Area jammers and nuclear "white-out" patterns are typically used to obscure enemy view of friendly penetration for just this reason
Wil correctly wrote that identifying jump destinations by watching jump entry is
possible. Sadly, I know too many of you will immediately conflate that
possible with
automatic.
When you read the passage I quoted, pay close attention to the many qualifiers used in the description. Penetration
can be detected. Detection requires a
hard fire control lock.
Three different sets of data, penetration angle, jump envelope configuration, and entry vector, must be observed.
Proper analysis
can allow a
prediction of
likely jump distance and direction.
Along with the many qualifiers used, pay close attention to how such detection can and usually is prevented or spoofed.
Observers can predict a jumping ship's likely destination if a number of prerequisites involving detection and analysis are met. Such predictions are not automatic or easy nor can such predictions be made beyond fire control lock ranges.