ora-1555 not in alert log?

I have a client/server odbc application (packaged from a vendor, no source) which returned an 'ORA-01555: snapshot too old: rollback segment number 7 with name "RBS08" too small' error. However, that error is not in the alert log? I see log switches subsequent to that time. 8.1.7 on hp/ux, a well-tuned production instance that has no errors besides expected ones from the standby db being down. SMON left a trace file indicating it cleaned up after problem, taking 70 minutes to do so.

I can understand the ora 1555, as some unusual processing is taking place as I try do some ETL while others are doing big processing, but what concerns me is why the error is not in the alert log. Yes, I've triple-checked the dates and instances, it happened today on the instance whose alert log I looked at.

jg

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