[imwillofficial]

Mental health issues can often be unpredictable and dangerous. A missing arm is almost never a sign of danger.

A missing arm could be the result of many things, birth defect, boating accident, or dangerous behavior, but is not a commonly accept indication of danger to self or others.

Mental illness is.

[MichaelMcG]

It's a sign they might have previously engaged in dangerous activities, maybe they learned from it and have an interesting story to boot.