وَقَدْ تَكُونُ لِلْمُفَاجَأَةِ، فَيُخْتَارُ بَعْدَهَا الْمُبْتَدَأُ، نَحْوُ: «خَرَجْتُ فَإِذَا السَّبُعُ وَاقِفٌ».
Translation: Idhā may express surprise; a subject follows it, e.g. “I went out, and suddenly a wild beast was standing there.”
Idhā can mark a sudden discovery; this is idha al-fujāʾiyyah. The source prefers a following nominal structure in this use.