There is a Plant called Fenugreek that gives off a scent similiar to what you describe as Maple Syrup. This is veru common plant to the area in which you live. If the dog goes romping and comes in contact with the stems that is what is causing the scent you are smelling. This plant is used as a sweetener and smells like Maple syrup. Joan
Sorry I did not address the Mushrooms that you mentioned "Lactarius Rubidus", which truely can also give the smell of Maple Syrup if the dogs get into them. That is another viable possibility. If all is negaitive will the Urine test, that is suggested below. I would definately assume that it is one of the two plants mentioned.
One of the other Experts Nancy H. also suggested a Urine check since there is a disease that can cause the Urine to give off the same smell. Maple Syrup Urine Disease is caused by Branch chain amino acids (valine, leucine, and isoleucine) congenital metabolic disorders. We like to cover all bases. Joan