well yes, brake fluid doesn't just dissapear so unless all of your pads are worn to the metal then you have a leak. Either way it needs to be checked out, brake failure is no joke.
Is it really bad to warm up your car, I thought that was really good for it, I sometimes do and sometimes dont - but it always drives nicer from the off when I do!
You shouldn't rev it hard when not up to temperature but neither should you leave it idling.
It's pretty risky driving the car to be honest. I mean if the leak worsened you would lose almost all breaking.
Only if its on the road... If its on your drive or private car park then its not.