This is a typical case of visual markup being used to describe logical concepts. If then (at a later point) it becomes necessary to to additional processing for those elements (e.g., adding an index entry in this case) it is not really possible to do this from within TeX/LaTeX/ConTeXt/... because the visual markup is internally also used to format bits and pieces of other elements. E.g., in case of
\textbf we might see this as part of a description label or inside a heading definition etc, and those shouldn't also get indexed.
TeX internally knows if a macro appeared on document level (as it keeps track of an input stack) but unfortunately this information is not available at the programming level (not even in LuaTeX as of now though Hans may add it in a next release) and I know of no other way to determine if a macro is called directly by the user or inside a definition of some other macro --- if somebody has a grande idea how to get that info I would be delighted to learn about it.
I therefore always recommend to mark up all logical elements explicitly for that even a remote chance exists that they need special formatting or processing one day. If you get into the habit of doing that it is not much extra work, but it saves you a lot eventually. All that is necessary up front is something like
and then use
\term instead of the direct formatting directive. At a later stage you can then worry about making the definition smarter if necessary, e.g., replace it with a definition that also adds an index entry beside bolden the argument.
So in this particular case the best bet is as already outlined in the comments:
as already pointed out by others.