Much aggravation has been caused by surface mount electrolytic caps that develop leaks, leaking the electrolyte onto the circuit boards and creating almost countless faults in a lot of consumer grade electronic gear, and a large number of camcorders.
Spotting electrolytics that develop external leaks isn't too difficult with just a good lighting source, in some cases. In a SONY VCR that I examined recently, when the head amp cover was removed, it was fairly obvious that the SMT caps were leaky.
Around the plastic base of the caps, there were dark speckles under the mask. The mask is a protective coating that's placed on the bare board after the etching is completed, and consists of a very thin layer of translucent sort-of paint, essentially.
When the electrolyte gets on the circuit board, it can creep under the mask, and since it's acidic, it can begin to dissolve the copper traces lying under the mask. This was what had caused the dark speckles that were visible thru the mask, around the bases of the caps in the head amp. For very fine circuit traces, the acid can cause enough deterioration of the traces to cause interrupted paths, or open circuits, but can also in effect turn the copper traces into resistors. Traces under and near the cap bases should be closely examined for deterioration.
To accurately assess the reliability of a circuit board with electrolyte damage, you will need a finely tuned, recently calibrated crystal ball.
The eletrolyte leakage may also be evident on the caps themselves, near the bottoms, below the crimp. The acid can attack the aluminum canister that makes up the covers of the caps, and in the case of the leaky caps in the head amp, the bottom edges of some of the aluminum cans were noticeably discolored from the reaction of the acid on the aluminum.
The acid in the electrolyte can also cause solder to have a dark color with a flat or dull appearance.
In cases where the electrolyte leakage is very limited, leakage can be verified by touching a hot soldering iron tip to the caps leads/solder pads, and detecting a fishy smell.
There are many accounts of symptoms/problems and repairs of equipment with failed, leaky surface mount caps in the archives of SER.
-- Cheers, WB .............