Why is there grease on my aluminum pans, which were stored in my oven and never used?

A year or so ago, I bought a set of disposable aluminum pans of various types and keep them in my clean, unused, oven, mostly one stack in and/or on top of each other, along with other pots and pans of various types (pyrex, copper, etc). In the rare instances that I use the oven, I remove the pans, putting them in a different room. After the cooking/baking is done and the oven has cooled off and been cleaned, I put the pans back in the stove. After months of non-use, I intended to bake in one of the pans, but, when I pulled out the cookware, I found that almost all the aluminum pans had a greasy feel on the inside, even those that were stacked under others. The only ones that escaped the odd grease were the ones at the very bottom. Because the disposable ware was cheap and corrugated, such that cleaning them didn't make sense, I tossed them into the metal recycling bin, and used my regular cookware. I never grease such pans in advance of use. I scanned the inside of the oven, thoroughly, expecting to see grease dripping down from somewhere, but, I couldn't find a drop, except on the pans! All seals are intact, so, nothing could have got in from the outside, and I almost never use spray oils on the stove, so, nothing could have got into or otherwise affected the interior of the oven. So, where did the grease come from?