Researchers came up with a warning symbol for this exact scenario
"In the aftermath of repeated incidents where the public was exposed to radiation from orphan sources, a common factor reappeared: individuals who encountered the source were unfamiliar with the trefoil radiation warning symbol, and were in some cases not familiar with the concept of radiation.
During a study in the early 2000s, it was found that only 6% of those surveyed in India, Brazil and Kenya could correctly identify the meaning of the trefoil symbol."
"In the aftermath of repeated incidents where the public was exposed to radiation from orphan sources, a common factor reappeared: individuals who encountered the source were unfamiliar with the trefoil radiation warning symbol, and were in some cases not familiar with the concept of radiation. During a study in the early 2000s, it was found that only 6% of those surveyed in India, Brazil and Kenya could correctly identify the meaning of the trefoil symbol."
https://en.wikipedia.org/wiki/ISO_21482