What notifications and emails do clients receive automatically?

What notifications and emails do clients receive automatically?

I'm doing work for a client's client.  Primary client doesn't want secondary client to have any info until completion.

This morning primary client emails saying "don't send other guy any docs."  Secondary client emails asking about 7 files that were sent to him.  I DID NOT send anything to secondary client. 

So my question is this: what in the ever loving universe did Projects send to the secondary client!?  This is a rather large issue because these are brand new clients and I'm not impressing them with this shiznit.

I have searched and searched, and I haven't found anything about Projects automatically sending notifications or emails to client users.  Please help!