Maybe the pilots simply forgot to turn it off?

Yes, it happens. A few years ago, I was on an AA flight from SJU to JFK. There was lots of turbulence and after some on-again, off-again seat belt signs, the pilots got on the PA and curtly ordered the FAs to immediately discontinue all cabin service, sit down and buckle up and for all passengers to remain seated until further notice.

Of course, by then, the turbulence had largely subsided. 30-45 minutes later, a passenger in econ gets up to use the lav. First Class FA sees this from their jumpseat and screams at the passengers to SIT DOWN NOW because the seat belt sign was illuminated and because of the dangerous turbulence.

I calmly pointed at the sign, which was not illuminated and had not been lit up for the previous 30-45 minutes or so. FA sheepishly calls the flight deck and reminds them to turn it on, which they promptly did.

Later, I told the FA that if they could forget to turn it on, it was clearly possible to forget to turn it off. FA agreed.