I accidentally took a sip off a glass of chardonnay, but only a small swallow and had a EtG test approximately 16 hours later will it be detected and will I fail?

That amount? 16 hours? Maybe, unlikely but possibly.

People have drank more and tested negative in less time, but there is a lot of unknowns. Hopefully you don't fail.

How exactly does one "accidently" have a sip of wine?

I can see it happening, forget they're forced sober and take a sip, social occasion distracted and having fun. I can see several scenarios