[Ger-Poland-Volhynia] How to find passenger lists returning to europe

[daryl]

In searching for immigrate relative I came across to strange manifests. 
On April 19, 1905 Karoline Harke came to the U.S. with 2 children, 
Emelie and Emma. On May ?, 1905 Karoline Harke came to the U.S. with 2 
children, Friedrick and Emma.

This confused me bacause of the similarities especially when, on both 
manifests, they were listed on lines 4, 5 and 6. And on both manifests 
they were traveling to the same address.

Turns out that their husbands were brothers. Both brothers were married 
to a Karoline. The families were previously living in U.S. and the wives 
apparently traveled back to their original homelands of Plock and 
Bobrowniki, I assume to show the children to their Grandfathers. After 
what I assume was a month or two visit (because they listed there 
previous residences as Bobrowniki and Plock) they returned to the U.S.

I found this to be very interesting and a bit of a mystery for a time.
My question is, how do I find the ship list from when they left America 
to return to Poland?