Providers that aren't in the same practice group can not see each other's information even if they are seeing the same patient. The only way this is possible is if they are submitting referrals to each other with your information.
If the providers are in the same practice group, they can see each others' notes if they have the 'Shared Patients' permission turned on. This is determined by the privacy levels of each practice group.
If you have concerns please contact your provider.