Being in ketosis means that your body's primary source of energy is fat (in the form of ketones). When you consume adequate protein as well, there's no need for the body to break down its muscle tissue. Ketosis also tends to accelerate fat loss --- once the liver converts fat to ketones, it can't be converted back to fat, and so is excreted.
Ketones will spill into the urine ONLY when there is more in the blood than is being used as fuel by the body at that particular moment.
You may have exercised or worked a few hours previously, so your muscles would have used up the ketones as fuel, thus there will be no excess. You may have had a lot of liquids to drink, so the urine is more diluted. Perhaps the strips are not fresh, or the lid was not on tight and some moisture from the atmosphere got in.
Some low carbers NEVER show above trace or negative even ... yet they burn fat and lose weight just fine. If you're losing weight, and your clothes are getting looser, you're feeling well and not hungry all the time .. then you are successfully in ketosis. Don't get hung up on the strips; they're just a guide, nothing more.
Read more about Ketosis & Ketone Test Strips http://www.lowcarb.ca/tips/tips011.html
Here are a few videos I made about being in ketosis!