If you suspect moisture on the outside of the pipe, just hold a sheet of notebook paper against it. You'll see the blue lines blur and form a water spot.

A leak in a Cat. IV vent could be fatal as these are under positive vent pressure. If this is flue gas condensate, it should be very acidic. If there are any drops, you could pat some baking soda to them and see if it fizzies or not. Also, look at the puddles on the concrete floor. Condensate will eat up concrete. Remember your 8th grade chemistry: acid + base > salt + water

A tech needs to inspect this unit and test with a combustion analyzer for CO spillage. With the vent running, you can use gas leak soap bubbles or even Windex or my trusty chemical smoke puffer.

HTH

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