Why is "Table" not in the list of "Source" objects that can be searched for a given string? Materialized view is pretty much the same thing, and it appears there.
Generally Toad only queries the all_source table for the "search source" group.
Tables are not one of the object types in that table.
On the other hand, neither are views and MVs.
However, table and MV DDL are simply CREATE statements and storage params (and SELECT statements, of course).