Forgot to add that the above is as long as the pregnancy is far enough along (atleast 5-7 weeks)
There have been a few cases of women who do not get positive results using a URINE test but a blood test and ultrasound would definately pick up the pregnancy.
No, I do not believe that is possible. The only way something like this could occur is if that pregnant women were extremely early in her pregnancy (2 or 3 weeks perhaps) so that the test is not yet picking up HCG levels and it is too early to see anything on ultrasound. However, once a women is past 4 or 5 weeks, neither scenario would be possible. A home pregnancy test can give a false positive, but a transvaginal ultrasound is very accurate.