the implied tag in that search is getting mapped to exposure.
We automatically do this for the top N tags on a site. Therefore when you search for
exposure triangle
you are actually de-facto searching for
[exposure] triangle
Because that's the 14th most used tag on the site.
edit: Our threshold for this "search word to tag" conversion is 25 questions minimum, which may be too low since we totally rewrote search. I went ahead and increased it to 100 questions minimum.