Fluid leak inside car

Do you have the foot pump version of the windshield washer reservoir?
If so you may have a crack in the billows and may have some fluid leaking....long shot.

Brake fluid dripping down the firewall on the inside?...you would know by now though as your brakes would be mush.

Can't think of anything that would be inside with fluid though....

Did you blow out the core with air to remove and remaining antifreeze?
If not it could be slow leaking out until it's empty....I'd think it would only be on the passenger side though.

I would take the carpet out and put some paper bags on the floor and try to find it that way.

Mop