You are not criticizing the OP, I guess, because you acknowledge their point, that it was meaningless, but it was entertaining to distract the Stasi. But you are criticizing the OP, because you think the Stasi were so competent?
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.