This is perfectly legal. The use of undercover officers are frequently used in many situations. The most frequent is in the use of the sale of drugs. An undercover officer witnesses a drug sale, and they radio ahead to other officers to have the suspect apprehended. There is nothing illegal about this. As long as the officers have probable cause to believe a violation has occurred they can stop you. And the officer's observation of you not wearing the seat belt gave rise to probable cause. It is somewhat strange that your police department would use its resources in this way to catch seat belt offenders, I agree, but sometimes the police want to reinforce in the public the importance of wearing seat belts.
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