Here's a description of bladder stones from the Mayo Clinic.  As in my case, they are difficult to detect and I didn't even know I was not passing all of my urine when they developed.

http://www.mayoclinic.com/health/bladder-stones/DS00904

Definition

Bladder stones are small masses of minerals that form in your bladder. Bladder stones develop when urine in your bladder becomes concentrated, causing minerals in your urine to crystallize. Concentrated, stagnant urine is often the result of an enlarged prostate, nerve damage or recurring urinary tract infections.

Bladder stones don't always cause signs or symptoms and are sometimes discovered during tests for other problems. When symptoms do occur, they can range from abdominal pain to blood in your urine.

Small bladder stones sometimes pass on their own, but you may need to have others removed by your doctor. Left untreated, bladder stones can cause infections and other complications.

Symptoms

Some people with bladder stones have no problems — even when their stones are large. But if a stone irritates the bladder wall or blocks the flow of urine, signs and symptoms can develop. These include:

• Lower abdominal pain
• In men, pain or discomfort in the penis
• Painful urination
• Frequent urination, especially during the night
• Difficulty urinating or interruption of urine flow
• Urine leakage (incontinence)
• Blood in your urine
• Abnormally dark-colored urine

Bladder stones come in a variety of characteristics:

• They can be small, or large enough to fill your entire bladder.
• They can be soft or hard.
• They can be smooth, or jagged and spiked.
• You can have a single bladder stone or several.

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