From Longman Dictionary of Contemporary Englishbe onto somebodybe onto somebodyinformala) (also get onto somebody especially British English) to speak to someone in order to tell them or ask them somethingA number of people have been onto me complaining about the noise.Get onto the Press Office and find out what’s happening.b)to know that a particular person did something wrong or committed a crimeThe police are onto him. →onto
Examples from the Corpus
be onto somebody• He's scared. He knows we're onto him.