I don't know where you get such notions. Any action by a company that affects share price is deemed 'material' and must be announced.
Here is what the 'public announcement' should contain, which must be posted before purchasing any shares.
"Should a company publicly disclose its share repurchase program?
Yes. In order to avoid potential liability for insider trading in connection with a share repurchase program, a company should publicly disclose the program prior to its commencement. Disclosure should be made after consultation with counsel. At a minimum, disclosure should be made with enough time to allow the market to absorb the announcement and include the following information:
the estimated time period during which the purchases will be made;
the maximum number of shares proposed to be acquired or the maximum amount of funds to be expended;
the objective of the acquisition of shares;
any plan or proposal relating to the disposition of the shares to be purchased; and
an indication of how the purchases will be made.
The disclosure may be made in a Form 10-Q or 10-K, or by means of a press release or Form 8-K, depending upon timing of the approval and commencement of the program. The company also should issue a public announcement disclosing any material modifications to a share repurchase program.
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