10b5-1 plans do not have to be filed with the SEC.
(The use of a 10b5-1 is also not technically a legal requirement, but is a workaround to allow planned sales even if you become aware of material, non-public information between the time of entering into the plan and the time of the trade.)
You can prove the boss had an intention to sell his stock based on a feeling of future doom (or just greed). What’s tougher to prove is whether he delayed an announcement by a week so one more sale went through before the stock took a hit.
Or speeding up good news so his scheduled sale rides the high.
(The use of a 10b5-1 is also not technically a legal requirement, but is a workaround to allow planned sales even if you become aware of material, non-public information between the time of entering into the plan and the time of the trade.)