Non-clustered Index Update not displayed in execution plan
Try generating an actual plan in SentryOne Plan Explorer.


  


For a narrow plan the actual costs are not included in the XML, so you won't see those, but you at least have the ability to see that non-clustered indexes are affected.

A lot more information found in the duplicate question:


Nonclustered Index Insert


As well as these blog posts that Martin pointed out previously:


Wide vs. Narrow Plans
Optimized Non-clustered Index Maintenance in Per-Index Plans