Is this coffee Kombucha? About three weeks ago, I brewed iced coffee in a clean jar; I didn't drink all of it and left it in the refrigerator.

About 1/2 the jar was filled, I then traveled and when I came back, I found in the bottom of the jar something sticky like rubber.

Is this Kombucha ?

I didn't use any kind of "scoby" aka starter, it's just ground coffee with cold water and sugar. This isn't the first time this happened. It smells just as coffee would, nothing bad.
In order to be Kombucha, you would need to have a starter, unless (somehow) enough bacteria was present in the coffee / sugar / cup (highly unlikely). It would also be extremely unlikely that even if a culture was unwittingly introduced, that it would bloom in cold water. It's not Kombucha unless it's actually fermented, and for that, you need a live, active thriving culture (you must have a starter).

What you've probably got at the bottom of the cup is a combination of settled sugars and residual grounds. While I doubt that tasting it would be harmful, I wouldn't quite recommend it :)