Setup system across from office entrance a month beforehand. Drop the tag into the target's coat at restaurant coat check. Trigger remote system based on proximity. Someone's probably already done this in a spy movie.
It'd be a much better idea to simply have a remote controlled system, using tech that's been around for decades. Though I suppose if you're truly paranoid, you might be worried about signals tracking. Maybe have it connected to the Internet and use thermite to wipe out the evidence after shooting.
