If you have dfhack, there is option to enter and find a name, type in part of the first name to narrow selection. If there is no one with that name, then that's probably a vampire or some other miscreant.
I have DFhack and I used the search (q) to check the unit screens, trying both first and last name (in case there is some weird letter) I also tried dwardtherapist (with nothing hidden and no filters) same.
I accepted the petition just for fun, still don't see the name anywhere. Also I use custom profession for everyone, so usually anyone new stands out and I don't recall anyone (forgot to check unit count before to compare.)
If the applicant doesn't "exist" it's someone who uses a false name, which typically means they're a vampire or a spy. However, if you accept a spy as resident->citizen the resident acceptance immediately severs the ties to the old civ, and creates a new one with yours.
If so then its a spy, because I just used the option to check if there are any cursed units and there are none.