There are plenty of external gadgets that extend the capabilities of an iPhone, Wildlife Acoustics has something called Echo Meter Touch 2, which when plugged into your phone can hear bats. And if it can hear bats, then it stands to reason that it can detect vampires in their bat form.
Seriously, though, this ultrasonic sensor can help wildlife fans find and identify bats by their specific echolocation call. It’s not cheap, but if you’re into bats, it’s a bargain. WARNING: Does not work on bat-like superheroes. Learn more here.