Questions & Answers about Il ladro è scomparso e nessuno lo ha visto.
Why isn't there a non before lo ha visto if nessuno means 'nobody'?
Why do we use è for the first verb (è scomparso) but ha for the second (ha visto)?
What is the dictionary form of scomparso?
The infinitive is scomparire (to disappear). It belongs to the -ire group of verbs but has an irregular past participle, which is why it becomes scomparso instead of following the regular pattern.