I can only speak for ketones produced by low-carb dieting, in my own case . . .
I found it quite difficult to find ketones in my urine, testing first thing in the morning. One day I tested mid-afternoon and there they were! I can only conclude, as far as ketosis is concerned, that it's possible to slip in and out of it depending on what and when you eat, and what and when you test.
Maybe have a day and test every time you go? Easiest when you're at home all day.
Don't forget that, on a very low carb diet, it is quite normal to show some ketones. This is ketosis - burning fat 'cos you're not eating enough carbs to produce enough glucose for the body to use.
Ketoacidosis is the very dangerous one; this is associated with high blood glucose levels. If you have ketones in your urine, high bgs and feel ill, get to the doctor asap.
Speaking personally only, I don't worry about ketones in my urine if everything else is okay. But then, I rarely show more than 2 shades up on the Ketostix. The main thing is to do whatever you feel comfortable with, and if in any doubt, go to the doctor.
Viv 8)