She's in Norway, where a "spot" might commonly be a social blemish or social embarrassment.
1 : a taint on character or reputation : blemish upon moral purity : DISGRACE, STIGMA, FAULT, REPROACH <keep this commandment without spot — 1 Tim 6:14 (Authorized Version)> <the only spot upon the family name>
A social blemish wouldn't be something brought up in a biology post. But, once you said blemish, that triggered it in my mind that maybe Thallium means acne scars...does spot mean the same thing as zit or pimple if you're speaking the Queen's English instead of our crude American English? It sure sounds better calling them spots than zits!