I would wait. Sometimes stuff disappears and then re-appears without me even doing anything! It doesn't always work, but it is less stressful.

I just wait to allow the problem to resolve itself. The missing item does seem to either reappear by itself or he simply forgets about the missing item. In the past I would try to help him find it, however, the search seemed to escalate the situation more in his mind, I find its better to just let it go.