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After some further research, I discovered that someone else other than coinb.in may have had access to a private key.
I will update here once I compile some proof and reveal the identity of the scam artist. Sorry if my original post implied that coinb.in was the perp as it doesn't seem very likely anymore.
That's unfortunate that someone else has access to your private keys.
But to confirm, coinb.in has not been compromised and that the source code found on the site and github does not leak your private keys. Aside from its thousands of regular users who i'm sure we'd hear from if there was an issue there are also honeypots in place and scripts regularly checking the files being served (https://github.com/OutCast3k/File-Integrity-Checker) which you can also setup.