What would cause a bruise with no known reason?

Abnormal clotting. Abnormalities of blood clotting mechanisms, e.g., low platelet counts, fragile blood vessels, low coagulation factors can result in bruising without trauma. If you are experiencing such symptoms, see your doctor.
Meds? Are you taking any nsaids, Aspirin or blood thinners like Coumadin (warfarin) or lovenox? These can thin your blood and cause easy bruising.