Another complication here is that adding mana would be something that wouldn't be visible, so I think it's assumed to have resolved unless you show you were unaware of it. I know with Prowess triggers, you don't have to show any awareness of them until damage happens. e.g. I have a 1/1 with Prowess, cast a spell, then attack. Opponent blocks with their 1/1. I just say "ok, so yours dies, but mine is a 2/2 so it lives".

So in the OP's situation, their opponent could cast something and still technically have mana floating. Or is it the case that since you have to announce floating mana, the lack of announcing it becomes "an action" in itself which proves you missed it, and oh no I've gone cross-eyed.