A symptom can be defined as any feature noticed by the patient, while a sign is noticed by other people. It is not necessarily the nature of a sign or symptom that defines it, but who observes it.
Donor plaques are meant to act as signs, showcasing the donor as an enabler. These plaques assimilate and become part of the architecture. The result is a symptom where only the donors themselves are aware of their existence.
Unlike the donor plaque, the cairn is anonymous, cost-free, and unavoidable.