I’ve wanted this detector for some time now. I watched a review on YouTube by HB CZ, and for all I know, you are him, due to the similarities. He was concerned over this very same issue. He states he was assured by Whites that this condensation issue was normal and due to physics, unavoidable. Said they used cold drinks in a glass (iced tea, lemonade, etc), and vehicle windshields as examples. I’ll give you one more example. Eyeglasses. Stepping out of an air conditioned building or vehicle into 95 degree hot air, and bam, condensation. The difference in temperature is what’s going to cause this effect, and I suspect your kitchen sink water was pretty close to room temp, thus no condensation. Same goes for when you were at the beach the one time there was no condensation. If the water is considerably cooler than the temp outside (or vice versa), you’re gonna get condensation. Oddly enough HB CZ also showed where Whites painted his coil with some type of waterproofing material. I digress.
He goes on to state that if the condensation does create an issue it will show itself before the warranty expires so he’ll not concern himself with it anymore. He said Whites assured him that the internal components are coated so that they are waterproof and corrosion resistant. To what degree and for how long is anybody’s guess, but I suspect any machine designed for use around salt water and beaches must have additional component protection.
My first thought while watching the video was they probably all have condensation going on given the right circumstances. The difference is other brand’s control boxes (Fisher CZ 21, Garrett Infinium, Tesoro Tiger Shark, AT Pro, AT Max, ATX, etc,) are metal or opaque plastic. You’ll never see the condensation but you can bet it’s going on.
You mentioned using other machines that never had a problem with leaking. Did your TDI actually have a leak, as in water pooling in the bottom of the control box? Did the detector malfunction because of it? If so, that’s definitely worth concern. Personally, the condensation issue won’t dissuade me from purchasing this detector. Don’t worry yourself. You have a very capable detector.