How was Peter involved in the resistance and the handkerchief?

In Chapter 17, Annemarie reveals that only a few months before, Peter was captured by the Nazis and shot in a public square. On the day of his execution, Mama and Papa told Annemarie that the Nazis murdered Lise as well. Lise and Peter had both been part of the Resistance. German soldiers raided one of their secret meetings and everyone scattered. The Germans purposefully mowed Lise down in a car as she tried to escape.

As for the hankerchiefs;

"It was Peter who took the problem to scientists and doctors. Some very fine minds have worked night and day, trying to find a solution.

"And they have created a special drug. I don't know what it is. But it was in the handkerchief. It attracts the dogs, but when they sniff at it, it ruins their sense of smell. Imagine that!"
Annemarie remembered how the dogs had lunged at the handkerchief, smelled it, and then turned away.
"Now, thanks to Peter, we will each have such a handkerchief, each boat captain. When the soldiers board our boats, we will simply pull the handkerchiefs out of our pockets. The Germans will probably think we all have bad colds! The dogs will sniff about, sniff the handkerchiefs we are holding, and then roam the boat and find nothing. They will smell nothing.