Furnace was acting up, had a tech come out and pull the fan and this was jamming it. My wife had just changed the filter, curious if she somehow knocked it in there? Lived here 8 years and have never seen this in or near the furnace. Tech had no idea either what it was either.
Made by Filtrete, looked like it was glued to something? Has a pull tab that maybe protects an internal battery? and a small circuit board.