Might someone have kidney stones, but not have any evidence of them in a urine sample or an ultrasound?

Yes. There is not always blood in the urine with a stone and ultrasound is really not the best imaging study for kidney stones.
Yes. Having kidney stones is detected by radiological means. Tis includes plain films, ultrasound, and ct scans. The gold standard test is the ct scan but it emits a lot of radiation. Ultrasound is also a good study but much more limited. It can certainly miss small stones in the kidney and will miss all stones in the ureter (kidney tube) because ultrasound cannot adequately visualize the ureter.
Finding kidney stone. Yes. The best way to find small stones is with a high resolution cat scan. They can be easily missed with ultrasound, and often do not cause blood in the urine on a consistent basis.

Related Questions

Is it possible to have kidney stones, but not have any evidence of them in urine samples or ultrasounds?

Yes. These stones may be too small see on ultrasound and a routine urinalysis may easily miss stones as well. The best test for diagnosing the majority of stones in the urinary tract is a ct scan without contrast. This will detect the majority of kidney stones but a few may be missed occasionally. Read more...
Kidney stones. Kidney stones may present as sand in the sediment of urine can only be detected when the urine is strained. Read more...