True
A participle is a verb that functions as an adjective. In this sentence "Disappearing just after dinner" is a participle phrase. Disappearing is a verb that functions as an adjective. It describes Ann and Chris as disappearing. One way to check and see if something acts as a participle or as a main verb is to remove the phrase from the sentence. Participles aren't required for the sentence to make sense. For example "Ann and Chris took a long walk on the beach" is perfectly fine by itself.