Sounds possible... returning before he was declared missing. Otherwise, once he was flagged, he couldn't clear back into Canada without being detained. 

Of course, there are many ways to cross the border without going through customs. As in, any small boat that can cross the Strait. Or walk across most of the northern border of WA. Most of the border up there doesn't even have signs once you get far enough away from roads. Canadian trees look the same as American trees.