There are times when I did not register anything on the cheap breathlyzers I have owned even when I have not eaten in 24 hours. My ketones rarely get high and I do not bother. I stop eating at 5 usually and often at night I am freezing, the same as when I used to do longer fasts. I assume I am in ketosis but as long as I keep losing weight I do not really care
I did get the keto flu back in 2017 when I first started. I have hashimotos so that may be the reason I really do not get deep into ketosis but I am not sure