[UltraMagnus]

Not open source, but I have used this before, and they have a very generous free tier: https://www.gitguardian.com/monitor-internal-repositories-fo...

You install their Github app and give them access to your Github repo (private repos are ok too) and they run a Github workflow when each PR is submitted scanning for secrets that should not be in the code. Really happy with how their product works.