Residue in SS Milk Can

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I had a weird experience today. I mixed and proofed my Rum spirit run yesterday, which came out very nice, in a stainless milk can. 3.25 G total product at about 115 proof. The can was solvent wiped and then washed with dish soap and plenty of rinse water prior to use. When I finished transferring the product to jars today, mostly to determine exact volume, I noticed a layer of something fairly transparent everywhere up to the fill line in the can. The dregs in the can were almost syrupy after sitting a few hours with the lid on (8 G can) in about 40 F garage ambient conditions. I did a very small rinse of dregs with a few teaspoons of water and taste tested the dregs and didn't notice any notable flavors.

Any ideas what formed this layer? It did not rinse out, I had to lightly scrub it out.

Thank you for any ideas!

NF