I'm still in the dark on this!

If I have an sp called sp__x in databases Master and NotMaster (and I didn't run sp_ms_marksystemobject), and I'm currently in NotMaster, then the sp__x that is executed is _always_ the local copy, i.e. the one in NotMaster. So where does the master database come into this? Surely if Sql server is always going to run the local copy of sp__x, then there are no search issues to consider. Or am I missing something here?