A well-kept secret has not been told or made known to anyone, or has been told or made known to only a small number of people
A well-kept building, street, garden, or other place is always neat and tidy because it is carefully looked after. two idyllic thatched cottages with well-kept gardens
{s} kept in good condition; well looked after; shipshape, neat, tidy, orderly (of a place); confidential, not revealed to to any person or to only a few people