Not all errors are logged in the message log.
try with 'grep -i error' to have the normal error lines in the message log.
The more severe errors are 'Assert Failures'
In this case it is possible that the server goes down.
It will write all evidence to an af.xxxxxx file before it does.
A list of all error codes can be found in the documentation section of the informix website:
http://www-3.ibm.com/software/data/i.../pubs/library/