Sagemaker Service

Your Cost explorer can help you find what exactly are you being charged for. You can filter by SageMaker, and then group by 'Usage type'. In general, make sure -

1. No notebook instances are running
2. No endpoints are being hosted
3. If you're using SageMaker Studio, all the apps are shut down for every user profile. There might still be EFS charges if you have data over the free tier. See this documentation for shutting down resources - https://docs.aws.amazon.com/sagemaker/latest/dg/notebooks-run-and-manage-shut-down.html

And, make sure to check all regions, if you've used multiple regions. This should stop recurring costs (if you had training jobs, processing jobs etc - you only pay for the billable seconds, so you won't see recurring costs).