"I realized that I did not bring my passport."
It is sentence final 了 that indicates a change of situation. Verb 了 is an aspect marker that indicates that an action has been completed. Verb 了 occurs with action verbs, where the action is taken on a discrete set items, such as "I ate two apples" 我吃了两个苹果. (Note: in some situations the action will be completed in the future, so verb 了 is not strictly a past tense marker.) I'd be curious to hear from native speakers if they ever use verb 了 with 发现.