Can someone on methadone treatment transfer it through salvia/kissing their significant other?

There is not enough to produce symptoms or turn that persons' drug test positive. I would not worry about it.

Methadone can be in sweat, semen, urine, stools and saliva and can be picked up with drug testing of these fluids in the person taking the medication. That is why they use urine for drug testing. Some medication can be passed through saliva to a partner but in such a miniscule amount. Not enough to affect the partner but technically if they immediately had a saliva drug test it could be positive.