Surely the point is that the OP couldn’t possibly have known what the authority thought about their painted stones, unless:
- they had a personal contact, which is quite the omission
or
- the authorities were putting up posters around town, interviewing door-to-door etc
Cops going around and asking about purple rocks would cause a tiny stir. A kid would be aware of that and entertained. This is pre- internet and It’s Soviet culture. I’m not supporting the Soviets, but people talked to each other, which is generically quite positive.