I seem to have a leak under my car on 2005 Buick LaCrosse

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I just had a trasmission leak and some other enigne leak fixed in the last month and the mechanic who fixed it assured me he checked for any other leaks and there was no other leaks. Well I am seeing another leak under my car it looks like water. It is the size of a half dollar coin. I checked all the fluids trasmission,oil,radiator and even windshield washer fluid and nothing is leaking. Should I take the car back? is this something new? do I ven trust the mechanic who did the work?
(1) Answer
Park in s diff. place to see if ANOTHER leak has developed.By ANOTHER i mean something else other than the mech.
fixed.There are a lot of things that can happen quickly to any automobile,even after a prof.
mech.fixed other problems.As far as trust let the same mechanic have another look.
because you have to trust someone unless you are car wise.