You are right in that poison sumac is not a common plant in our area and prefers swampy areas, , whereas, staghorn sumac is very common and will pop up in almost any terrain.
However, we can't confirm that your volunteers are, in fact, sumac. The foliage is not similar to what we would expect of staghorn sumac and we cannot see the flowers on your photo.
It is quite possible that your volunteers are actually black walnut volunteers. You could try bruising a leaf and smelling it. If it smells like rancid peanut butter, it is black walnut.
Keep in touch.
Let me correct myself. The rancid peanut butter aroma is more associated with the "Tree of Heaven", which is another sumac 'look-alike'. Tree of Heaven, Ailanthus also has smooth leaf edges while sumac has toothed leaf edges.
Your volunteers are actually most likely Ailanthus.
Thank you for the great information!