I what appears to be a pocket of fluid, around 8 cmm. in the upper right quadrant of my abdomen.  It is visible as a slightly raised area versus the other side.  The exact term from my doctor was R Subcostal fluid buildup.  It appeared suddenly last Sunday evening or  Monday morning, around the time I went for a sonogram to check out what my doctor thought might be a periumblical node or cyst.  That turned out okay - a cyst or fatty tissue,  all of my organs showed clear as much as could be told by a sonogram.  But, the radiologist saw the fluid build-up; he kept asking me if I had injured myself, I didnt think I had although I had been sweeping up leaves the night before and stretched pretty aggressively a couple of times.  I was nervous the morning of the sonogram and was holding myself very rigidly.  I also had to go to the bathroom.
Hi. The built-up of fluid in the R-right subcostal area is definitely dangerous as this is never a normal finding. One has to get to the root cause, which can be due to Cholecystitis, can be a cyst, can be related to any of the inflammatory process and can be tuberculosis. I would advise undergoing CT scan to confirm, get a Consultation of a General Surgeon for clinical examination and further investigations and treatment.