most helpful answer an ultrasound is only as accurate at the technician doing the exam so yes things can be missed. a cystic mass (spermatocele) should be able to be seen but if the spermatocele is calcified and no longer filled with non-viable sperm it could be easily missed. spermatoceles are usually no big deal if you are not having symptoms but if you are concerned see a urologist for a higher-level evaluation.
can an ultrasound not "see" something? i have a pea-sized spermatocele that wasn't found on an ultrasound. my doctor told me to get an ultrasound of my testicles after he said i have a spermatocele. but after a week with no call back i called his office for the results. his nurse said that nothing was found. i asked for clarification and she said the same thing. so i explained what it was for and she put me on hold to ask the doctor. she came back and said the doctor told me to take pain meds if it hurts. i expected a more definitive answer like "it's fine" or "it's bad" not "there's nothing there. "