Rule reminder
Consolidation: vedere/sentire/ascoltare/guardare take a bare infinitive for direct perception. Accorgersi/rendersi conto take 'di' before nouns and infinitives (same subject). Notare takes nouns directly or 'che + indicative'. All take 'che + indicative' for indirect/reported perceptions.
Suddenly I noticed someone watching me. → All'improvviso ho notato qualcuno _____.