Definition of be onto someone in US English:

be onto someone

phrase

informal
  • Be close to discovering the truth about an illegal or undesirable activity that someone is engaging in.

    • ‘The cops were onto us and there was absolutely nothing I could do about it.’
    • ‘He figured Uncle Byron must have known that Devlin was onto him.’
    • ‘She'd been contemplating whether or not to let Brandon know she was onto him.’
    • ‘More importantly, how did Renee know she was onto her?’
    • ‘He must have sensed that she was onto him, because every time she would try and approach him, when they were camped for the night, he would just look at her and get up and walk away.’
    • ‘What he most feared, however, was that Elsa was onto him.’
    • ‘She must have realized he was onto her, for then she relaxed herself and allowed her soft pink lips to straighten out, forming a grim line on her face.’
    • ‘I didn't want her to even guess that I was onto her.’
    • ‘They thought they were onto somebody who was robbing the house.’
    • ‘When Jackson realized the family was onto him, he tried to send them to South America.’