I thought I updated this....I gotta look back. The 2016's DO COME WITH plugs....After the 3600 serials start...Mine is a early 3500 serial and no plugs. Dealer has another 3600 with them...Factory installed.
Of course I wash it with a hose, and a normal wide size nozzle. I dont insert the F-ing thing in the hole, but I am pretty sure of all the bikes I have owned they are all waterproof and drivable in the rain. I looked all thru the owners manual and no where does it say "dont get frame wet!!".... (Ok, I didnt really look, that was for emphasis)...
Well technically, its some of the 2016's come with plugs. LOL. I never got serial of 2016 my dealer had. I just looked to see if the weld holes were and they weren't. So that would mean it was an early serial/frame.
Hey, wash your bikes how you see fit. I was just making an implied statement that with a hose, water will travel further, more pressure, and can be sprayed at any angle. It's totally separate from the angle rain comes down. Sure say the water spray from the ground can get in the holes. I'm sure it can, but not as easily as a from a hose. I've actually been caught in rain on mine and very very very little water, okay practically none made it into mine. Reason, on right side, front hole has clamp in it and its partly covered by motor. left side is more exposed but due to my legs and loose fit jeans, most of the road spray hit my leg before the frame in that area. SO, hose spray in your case is the cause. Is it rusting due to lack of Kawasaki rust treatment getting up in there?... maybe. But its a risk most take hosing off any bike for that matter. Just because you cane see rust doesn't mean it's not somewhere. I know and except the rain may have gotten in mine and I just haven't seen it in the typical location.
However, I don't make it a point to ride my h2 in the rain or hose wash/spray it. If caught in the rain, I make it to the nearest underpass or covering and wait it out. If I have to chance riding in the rain, I have 2 other bikes I'd chance and ride instead.