After work, I brought my car for radiator check

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Followed by today morning's discussion, about the radiator leaking problem I have. I brought my car back to the Shop along with the receipt, and have them check for the problem, why is that every time I fill up the coolant in the reservoir, next day, it gone down to low. So, I suspect there is a radiator leak, since they installed a new radiator on my car about a month ago. Now, the mechanic lifted my car, and check under hood for any possible problem. But he couldn't find any leak under it, and show me too. He told me that maybe some of those coolant circulate around and may not coming back to the reservoir, and that is why it seems empty all the time. He also remind me to check the tailpipe for any smoke, or water flow. If that happen, it is the heat air something like that. Would that be possible? Does anyone give me any advise on this problem? I am totally a dummy of this car repair thing.