Acetone didn't budge the marks; I'm trying to decide if I care enough to pop out the panel. It appears the previous owner stuck it in place with gobs of sealant. It's in a useful place, and I'm not sure I have the dexterity to replace it if I take it out.
Meanwhile, I'm removing the old sealant from the sides and bottom. The scraper I'm using was labeled a "handy scraper" at the Rite-aid, and boy, is it handy. Even so, it's a difficult job.
There's almost no sealant along the side seams of the tank. I'll probably be a bit more generous. There's no need to be messy, but stinginess might've created that suspected leak.