I'm inclined to blame the hot water episode - it was VERY hot -because there is no obvious impact mark, just the hairline, and it's close to the taps. On the other hand, flexing might do it, as it's near to where I stand to shower. Second thoughts - the relation to the position of the legs isn't right for the forces - I'd have to be balancing on one leg over the plughole, and I don't do that. I think the previous owners had had some liquid in there, probably on just one occasion. The level is below the overflow, and there is just one tideline. Whatever it was did not damage the metal plug surround, which just shows signs of wear, not corrosion.
I've actually been in the house some time, but moving from bedsit land had a rather high threshold of tolerance, and a rather low level of interest in the oddities left behind. As I change the carpets and so on, I am finding that they didn't just have terrible taste in wallpaper, they did things oddly, too. The master bedroom had white shag pile carpet. Below it, most of the floor was bare floorboards, but on two sides only had a strip of vinyl. A piece of this vinyl had been used on the landing as the centre part, surrounded by 1950's coloured linoleum tiles, with no obvious pattern to the laying of three colours. Where is the rest of it? (The house is 1960's.) Under the bath is a shallow biscuit (cookie) tin to catch drips from the cracks in the surround. Not sufficient with a shower, as I found out in my kitchen one morning.
I was very glad to find out how innocent the things which can strip the top layer of the enamel can be!