You could be confusing diacetyl with sulfur as well. I have known people to not be able to make the distinction.
And yes, diacetyl is formed during fermentation, but once the yeast finish up their attenuative phase, they will reabsorb vicinal diketones which are the precursors to diacetyl.
Easy way to tell if your beer has reabsorbed VDK is to take like 200ml of your beer, heat it to 140, hold it for 90 minutes there, then let it cool to room temp and drink it
you are going to have a shitload of weird flavors, but you should be able to pick out diacetyl pretty quickly
Ohh and dont use wyeast
SIebel Institute just did a study on wyeast strains and determined that like 30 percent of their strains are not what they say they are
in fact, their Saison strain is actually a wine yeast, that required an 85-90 degree fermentation temperature after like 3 weeks to actually hit its terminal gravity
white labs all the way