I found the same thing.  Without the leaves it makes it very difficult in the winter to discover.  The picture section has some good pictures of the vines which should help.  In my region (virginia) it is very common for the poison ivy to grow near Christmas trees.  Many of the freshcut farms will remove the vines just prior to sales season which makes it even more difficult to know. You can generally tell by observing the area around the trees because they don't remove the vines just cut them.