Are Amazon Echo ads embedding an ultrasonic signal to suppress recognition of the Alexa keyword?
Most speaker/amplifier systems will have analog filters that cut off far below 24 kHz. 

In fact, with 44100 kS/s being a very common sampling rate, you can be certain that you can't transport any signal at frequencies above 22.05 kHz; Nyquist doesn't only apply to the digitizing process, but also throughout the rest of the signal processing chain. Since digital spectra are periodic with the sampling rate, any digital-to-analog converter must have so-called reconstruction filters that cut off at half the sampling rate on each channel.

In other case, what your phone can't hear is something that your TV cannot emit, either. (At least not intentionally – analog filters, especially affordable ones, are far from perfect)

My guess that the software simply got better at determining the "intonation" of commands. Luckily, for all Latin- and Germanic-based languages, the imperative is not that hard to discern acoustically – if I had a guess how most of an infant's and a dog's understanding of human language starts, it's that – and you can tell someone commanding "Alexa, do my dishes!" from someone noticing it in the center of a sentence as object like "I can't trust Amazon enough to let Alexa listen in on confidential conversations."

Another relatively common thing is Steganography – in the case of Audio that's usually called Watermarking – the art of hiding info in a signal in a way that is recoverable, but not noticeable to the observer. It's been used for easily one-and-a-half decades now[citation needed] to track bootlegged copies of movie recordings etc.