Obviously something caused this to ferment. If your smell/taste test detects no funkyness I would guess yeast, without plating it there is no way to be sure though. The teqnique of a cotton ball, dipped in Alcohol and rung out, is a steril way oh capping while alowing gas to escape.Im inclined to blame the sampling method vs overall contamination though. Perhaps you might want to send a sample from that batch in for plating if you cant do it yourself just to be safe.
It may taste ok, but you may be building your contaminate load and get poor results from subsecuent pitches. Which is the purpose of this test, to quickly determine the sanitaryness(?) ,before the batch is done, for repitching.
Forced fermentation is a different technique used to determine maximum attenuation so you can gudge the progress of the batch.
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