I have a friend something like this happened to during the Clinton years. And he was absolutely uninvolved. Someone, aparently trying to be funny or get even or something, wrote a threat against Clinton on a dollar bill and put his phone number on it. He was thouroughly investigated by the SS. From what he said, they aren't a particularly friendly bunch. Fortunately, the handwriting was nowhere near what his was. I don't think the SS cares much about who is POTUS, they just persue their job with extreme vigor.