P0420 appeared, cleared, has not yet reappeared. One-off, or cause for concern?

Last week the dreaded 0420 code popped up. I had not ever seen it before in a year of owning the car. Logged it, cleared it, and have been expecting to get a repeat. So far it has not. 146K miles.

This code coincided with a change of OBD app. A couple of times in the past, switching from Hybrid Assistant to Torque Pro and back has triggered the random error code that cleared after I restart.
Do you think this could be a similar misfire or is a cat problem typical for a Prius close to 150,000 miles? Thanks.

 

So far, it hasn't returned. Thanks for your feedback.