Sorry I don't have a picture but if some names are throw out I will google to see if there is something that matches. A friend found a maple sapling coming up in the woods near her house. She lives on Pilchuck tree farm property (N. Snohomish county) so it is all 2nd to 3rd growth forest. The leaf I saw is about the size of my hand (5-6" across) much rounder than I expect for a big leaf, very dark green on the top side and the underside was a dark greenish, magenta or purple. We are just curious as it is not a variety seen volunteering around here.
Acer pseudoplatanus f. purpureum.. Not as bad as Norway maple but still "weedily naturalized" (A. Jacobson, Wild Plants of Greater Seattle - Second Edition, 2008) - I've seen the early stages of an entire grove coming up under alders on Vashon Island.
Thank you very much Ron. Does that mean we should let the tree farm management know :) ? This is probably at the road edge of established DF-Hem stands.