WHEC hasn't "signed off" officially since the network swap of 1989. Unless this was recorded during the Olympics, in which if that's the case, or if there was somekind of emergency like the attacks on the WTC it would temporarily sign off and then sign on a few hours later.
Wow, why would an NBC affiliate sign off? Hasn't NBC pretty much been a 24-hour fixture everywhere in the U.S.? The idea of a "broadcast day" is... alien to me.