The applicable legal terms are: "reasonable," "relevant," and "material."
The seller must make a reasonable assessment of any and all factors that are material (of significant value or risk) and that are relevant (that relate to the business after the closing) to the transaction, EVEN IF the buyer has not specifically asked the "right question." For example, knowing that the street in front of the store will be closed for repairs for three months shortly after the closing and not revealing that information to the buyer could be both relevant and material. For another example, knowing that the city may be planning on requiring shop owners to display street address numbers of a certain size at some time after the closing may be "relevant", but it is hardly "material." And, for yet another example, if the seller has terminal cancer and there is no significant way this will adversely affect the business after the closing, that health issue is neither relevant nor material and does not need to be disclosed.
In short, this is a "CS" issue -- common sense. If the seller has any doubt or question about material and/or relevant disclosures, this would be a very important conversation to have with an attorney, before entering into contract.
And, then there is the question of "When to disclose?"-- It should be sooner rather than later. Definitely any disclosure should be made during due diligence if known, or as soon as the seller does know if discovered after due diligence. The longer the delay in advising the buyer, the greater the potential damages to the buyer, and the greater the potential liability to the seller.