When the connection string contains slaveOk=true as an option then the following line of (groovy) code may read from a secondary server in a replica set:

collection.findOne(new BasicDBObject([_id: key]),null, ReadPreference.PRIMARY)

I believe that this may well affect all find methods.

This appears to be due to the code in OutMethod.appendQuery() which ORs in Bytes.QUERYOPTION_SLAVEOK if the read preference is NOT primary but does nothing if the read preference is primary.