I'd say with 1+ blood (as compared to trace blood) that it probably was there, but gone by the time you had it rechecked. The dipsticks are meant to be sensitive, but the results used in context, and repeated if need be.
I'd check with your AME, but there are a lot of causes of blood in your wee, including not having had a pee for a long while before producing the specimen. (Bladder, gets overfilled, and tiny blood vessels get stretched and burst causing blood to leak into the wee) Being on your period or near the end of one (if you are female) Having an infection. Kidney or Bladder Stones. Cancers. And sometimes no-one knows. Some of these causes may only cause blood on a few occasions, therefore it could have gone away when you get it rechecked.
(Check it out on the NHS direct website or
www.gpnotebook.com)
A better way to ensure you get a good specimen is to drink plenty of water, not hold onto your wee too long ( I always ask for the specimen bottle at reception - so I don't have to wait), and to catch the wee midstream so to speak (more difficult than it sounds); to make sure that it doesn't contain any contaminants from the pipes from the bladder.
If it happens again and again I would be more concerned that you haven't got anything nasty rather than the hassel it causes you.