I have checked all my forwarder and indexer and search head apps. but unable to find from where a field it's extracted.
The field extraction can be set based on sourcetype, source OR host. So make sure you're looking for all relevant stanzas for your sourcetype/source/host. They can be extracted (EXTRACT/REPORT/TRANSFORM in props.conf) , calculated (EVAL in props.conf) OR derived (using lookups).
Do you see the value of the extracted field in your raw data?
How did you check all the apps? Manually, or by running btool?
Might be helpful if you provide a bit more info of what you are investigating...
Are you using any app or add-on for data indexing?