DNS: we're not hosting a forward lookup zone, but getting answers for that domain.

I have a situation that I can't seem to explain on our internal network.  

My DNS (Active Directory) does not host a zone or any records for our internet facing domain. Yet, when I do Lookups in that domain, I get the internal address.

Example:  

Nslookup server.domain.com internal.dns.local
Answer: 192.168.100.xxx

nslookup sever.domain.com internet.dns.com
Answer: 12.xxx.xxx.xxx

How can this be if my DNS is not hosting that domain or record?  There is no entry in hosts file either.  And this works for multiple hosts on our internet domain.  

Thank you in advance for your help!