What can happen if I was ran up on by the police, had coke in my mouth and swallowed it and they didn’t get it?

If they have no drugs, then they have no case.

You should never talk to the police, just say I want the questioning to stop until my lawyer gets here and not one more word. Most people talk to the police and give them what they need to convict them because they are not very informed about these matters. If they suspected that you swallowed drugs they had no proof until you admitted it. If they let you go you are lucky.

Without evidence there is no case even if you admitted it.

If they let you go and did not recover the evidence, then they have no case.

To properly answer your question would likely require a thorough examination of the eventual police reports. The prosecution cannot convict you based on a confession alone (based on the corpus dereliction rules in Washington State) so there would need to be some additional form of evidence of possession to convict you. If the police can testify to some reason to believe you had cocaine, and that they observed you with a white, powdery substance prior to you placing it in your mouth, they would be getting closer to having a provable case. As such, it is very fact specific.