Wash and dry it off then watch closely. It will probably be most likely to leak as the engine is cooling off. You could try to pull off the rad cap with the engine cold and watch it. A pressure tester and dye might help as well if you still can't find it for sure.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)