How to prevent slaves from contacting master for name resolution?

[Keith Christian]

Our redundant DNS configuration is one master and three slaves, spread
across two colo facilities.

master and slave1 are in colo_ALPHA.
slave2 and slave3 are in colo_BETA.

During an extended maintenance window, the master DNS was offline.
Slave2 was trying to contact the master, and lookups failed.  Usually,
slave2 resolves without contacting the master, but occasionally it
does.

The IP for the master does not appear in slave2's /etc/resolv.conf,
and I'm not sure what else to check for on slave machines.  Where else
would I look?  Would any settings in named.conf account for this
behavior?

Versions are Linux (CentOS 5) and BIND 9.5.x.

Thanks.

======Keith