Interestingly, the western conifer seed bug (Leptoglossus occidentalis), is, according to this site (which refers to the bug as the pine seed bug), often misidentified as an assassin bug or a stink bug. Especially interesting because one of the links that homebird found and kindly left in the comments was to a page that shows a picture of the seed bug, and indentifies it as stink bug. Further investigation of this site reveals that there is also a page dedicated to the western conifer seed bug, with a different picture of what sure looks like the same bug. (I've sent them an e-mail about this.)
The University of Guelph Pest Diagnostic site has a page about this bug as well, one that goes into more detail about what a western bug is doing in eastern Ontario, and about the bug's structure:
Leptoglossus occidentalis is a member of the family Coreidae, the leaf-footed bugs. As such, it has the characteristic swellings of the hind tibia.This feature of the legs is clear in all the photos, mine, and those on the linked sites. The feature is absent from the collection of stink bugs at the other page homebird found for me.
Still, I'm amazed when an individual bug species is identified at all--there are so many, and the appearance of different members of the same family can differ so radically, in both their adult and earlier forms. I can't wait to see what turns up in the house next!!