Hello Jay 'Missing' is a genuine adjective meaning 'being out of the sight' (a state), whereas 'disappearing' is an present participle meaning 'going to be out of the sight' (an action or a process). When you use 'found', the construct is usually 'someone found something in a state' or 'someone found something'. So in the case of the problem in hand, you have to either 'I found my watch missing' or 'I found (that) my watch had disappeared'.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.