For the majority of cars that lose fluid without marking their spot, the problem generally is a bad modulator assembly. Now here is the issue, modulators have become outdated in the quest to computerize car transmissions. In the case of a car that shows no leaks at all, there is a good chance that your car has a modulator and the only issue is a bad modulator, which is a very affordable repair if you don’t keep running your transmission low on fluid.
What happens with a modulator is the diaphragm inside ruptures from old age or a good old fashioned backfire from the engine. A modulator uses vacuum from the engine to send a signal to the inside of the transmission so it knows how hard the engine is working, and adjusting the tranny and engine to synchronyze with each other so the transmission shifts properly.
When the diaphragm ruptures the ATF is then sucked by vacuum into the engine and gets burned and goes out the exhaust. Generally speaking, there is little or no evidence of a driveability problem, except for maybe some black soot in the exhaust pipe.
The only other way I can think of that fluid would be lost if is someone had a grudge against you and snuck up late at night and had actually drained fluid from it. Since that is very unlikely, you should have a qualified professional; transmission mechanic examine the problem. You may be dripping fluid, which is falling on an exhaust component and burning up, leaving no evidence.
One way to avoid some of these problems is to have your transmissions serviced at the proper intervals. Problems like this can be found and solved before you end up in the market for a replacement transmission. Call GotTransmissions.com @ 866-320-1182 and speak with a pro.