My guess is galvanic corrosion. Aluminum can cause corrision when in contact with Stainless steel and a liquid. I bought a used Masterbuilt Smoker that came with a Stainless water tray, and it had a number of pinholes in it. My unsupported guess is that moisture got between the stainless and the aluminum, and over time caused the pinholes, though I would have thought it would have caused pinholes in the Alumimum . Like the first time i put aluminum foil on a pan of lasagna - and went to take off the foil the next day and wondered why the foil looked like swiss cheese. If there is no moisture between the aluminum foil and the pan, there should not be any corrosion issue.