In jobs like what you describe, one approximation of "after hours" would be "when you're not working or at the office and are not in violation of a reasonable expectation/policy to be doing one of those things."

At some employers this would also include "present at the office at a time when you're not expected to do work."

In reality, it would be up to the judge, jury, arbitrator, or hearing officer to apply this phrase to the facts or circumstances of any dispute (possibly with deference to on-point precedential or persuasive case law).