It also happened to Lennard Zinn, who writes for Velonews.
IIRC, he felt something was amiss, as well as seeing his HR indicating >200.

A couple years ago in an EMT class, everyone was paired up with a partner, taking each others HR. One girl couldn't believe what she was finding on her partner, and told the instructor it was ~150. The instructor insisted that was not correct, then came over and checked for herself; The guy had an HR of 150 just sitting there. This was a guy in his 20's who had no idea anything was wrong. Last I talked to him, they were sending him to a specialist, but no diagnosis had been made.