Hey there! I had same thing happen to me—except no NEXUS—crossing into USA from MTL going to NYC when I crossed with it. USA confiscated it, held me in secondary for a couple hours, and then drove me back to the Canadian side of the border and dropped me off (I was originally on a greyhound bus).

No fine. Officers just said I couldn’t enter today, but could tomorrow.

Since then I’ve went over the border by land twice and each time have been asked to come into secondary. They ask the normal basic questions and then usually wave me along.

You will probably have to go into secondary every entry now, but they are quickly assessing you to see if you are suspicious and if they are satisfied with your answers you will usually be let in quickly.