Brass / Rust Contact

All I can offer is that brass on a baking sheet (dissimilar metals) plus water and heat seems a prime e3nvironment for a chemical reaction to take place, resulting perhaps in what you are seeing.

You may be seeing oxidation transfer in the form of rust from baking sheet to brass as brass will not "rust" per se.

I myself have only experimented BRIEFLY with wet brass in an oven and never did it again for the spots and stuff it left me with.

Since then decades ago, I always went from tumble into flash hole inspection then into the reloading process. (the brass was sized, decapped and where applicable belled prior to tumbling)

Are you rinsing them in water after tumbling to remove media dust? If so, there are better ways to reduce excessive media dust, that doesn't involve exposing your brass to water. I either add a dryer sheet, a shredded cotton ball, or a small piece of cheese cloth, in my tumbler. My brass comes out with barely a detectable amount of dust on / in them.

For my high power rifle brass, which I'm really picky about, I give them a bath in acetone after the final tumble. I do this only with bottle necks, because they get exposed to lube on the inside, when I lube the inside of the necks. And although this may seem extreme, I've taken a Q tip to the web region after a long tumble, and there is caked media dust still in there. And in my thinking, if it's caked, it's probably lube residue that contributed, which could transfer to the powder in the form of a vapor over extended periods of time.

GS

If you want your brass to dry quickly, final rinse it in very hot water with a teaspoon of Lemishine per 5 gal of water. Let the brass stay in the hot water long enough to heat up (10 - 15 minutes). Drain the brass, roll it in a towel, then transfer to a dry towel. It will dry in just a few hours in a reasonably dry environment. In the sun it will dry even faster. No need to use an oven.