If it's leaking fast enough to significantly go down overnight (presumeably with the engine off), it must be quite a substantial leak.

But where? Coolant can't just disappear* ...
[* it can be vaporized, though, if it's leaking into a combustion chamber, but that would only happen if the engine was running, and I'm sure you didn't leave the car running overnight]
... so when you write that you cannot spot the leak, I have to ask if there is, at least, a big puddle underneath the car? I sincerely hope so, because if, instead, it's leaking into the mechanicals (engine via a bad head gasket, or transmission via the heat exchanger in the radiator), you've got a really expensive problem -- engine coolant is not supposed to mix with engine oil or transmission fluid, and such mixing will cause tremendous damage if not stopped early!

And along with those questions, have you checked your engine oil and transmission fluid levels (and color) on the dipsticks? Should we assume that you already have, and they are normal color and at the proper level (not overfilled)?

One less serious alternative (in terms of repair bills, but still a big headache) is that it's leaking into the passenger compartment (via a leak in the heater core) -- don't forget to check the carpeting under the dash and your seats (although you would most certainly smell the antifreeze).

You wrote that you just bought the car -- this may be the reason why it was put up for sale. Do have any warranty (it is only 2-3 years old)?

Write back and let us know....