Well, I am spending a lot of time at my parents' and my husband has had all the tanks to maintain. But we are pretty sure it was overnight.
The chest is smooth porcelain - no edges, all rounded and finished. I just added it Tuesday. So I thought it was safe and can see how I may be totally off.
The spot on his yellow "cheek" might be underneath. The other one is open.
Thank you, jlk. You're awesome.