@brinonmiguel I believe the feature of dragging a sheet off the workbook is no longer available in newer Excel versions. It was possible, in the past, to drag a sheet to, for example, the desktop and you'd end up with a new workbook there. So if you are using an older Excel version, your "lost sheets" may sit there. Nowadays, one can only drag and drop sheets between workbooks. So, if you are using a newer version, could it be that another workbook was open and partly visible behind the one you were working in and that the sheets were accidentally dropped into that one?
Anyway, on a Mac myself with the most recent Excel version. I did not succeed in dragging sheets off a workbook, other than to drop it into another.