12.E.6
6. A reserve pilot will be released upon checking his HI3 for additional
assigned flying or by contacting crew scheduling in accordance with
the following:
a. A reserve pilot will be released from duty upon checking his HI3
after aircraft block-in if no additional flying has been added to his
schedule. The pilot may also call for release. Additional flying
may only be added to such pilot’s schedule, pursuant to
paragraphs 12.E.1 through 3 above.
b. A reserve pilot on RAP who has been called out for a flight
assignment ending that same day may also utilize this method
for release, but must also check his HI6 to determine his status
(returned to RAP or released to rest; the default is “released to
rest” if no HI6 is received).
Like a user above noted, regardless of how you checked the HI3/HI6 if you have proof of when you did with a screenshot you'd probably win a grievance if the company even decided to push that far.