First off .. and i think you already know this .. please dont taiste any fluid .. think battery acid ! nothing to say it isnt that .
Antifreeze in any reasonable amount will make you go blind .. we used to do this back when i started .. even i dont do it these days .
Ok .. your problem ..
I assume it wasnt there when you started so to me the act of cleaning the carpet has drawn this substance to the surface ..
Back in the day i used to be a dab hand with a wet vac and this did happen from time to time .. dont even ask me about the granda est that a zoo owned and what happened after i wet vaced the carpet !
So .. i think its something in the foam under the carpet that has now been drawn up to the surface .
As to what .. something previously spilt ..maybe ..
We are by the seat but if you move forward you have the pedals .. brake / clutch if fitted use brake fluid .. it does sometimes happen that a master cyl will leak inside the car .. fluid will collect under the carpet .
I think if you had a problem then you would know by now due to lack of brakes or clutch .. so its possible this is an old leak.
You are only left with pulling up the carpet and looking at the foam .. or not cleaning it again
just my 2pennys anyways .