Breakdown of watashi wa ijou o mitsukemashita.
Questions & Answers about watashi wa ijou o mitsukemashita.
は marks 私 as the topic of the sentence: as for me, ....
So 私は異常を見つけました presents the sentence as talking about me and what I did.
If you used が instead, it would put more focus on I as the subject, as if answering a question like Who found the abnormality? In many situations, は is the more neutral choice when introducing what someone did.
を marks the direct object of the verb. In other words, it shows what was found.
- 異常を見つけました
- found an abnormality / found something abnormal
The verb 見つける is a transitive verb, so it takes an object marked by を.
