In message AES
was claimed to have wrote:

During the departure taxi phase on a United 777 at Dulles yesterday
(4/28), the cabin crew made the usual safety announcements and
instructions that all electronic devices must be turned off until the
plane had climbed above 10,000 feet (or whatever is the specified
altitude.

A few minutes later, one of the cabin crew (at least, I thought it was
the cabin crew, not the cockpit, but can't be sure) came on the PA
system and said something like "Our signal radiation detector [or some
such term] is indicating that an electronic device in the cabin has not
been turned off. If this device is not turned off, we will have to
return to the gate and that passenger will be removed."

It's entirely possible one of the flight attendants spotted something
and lied about how it was detected just for giggles.

It's also entirely possible likely that they have some piece of
equipment which reacts to certain frequencies. A iDEN or GSM phone near
an improperly shielded speaker or amplifier can cause some interference,
for example. It's possible that some non-critical system reacts
similarly, creating an unintended (and likely unreliable) detector.