You might find your cat in the bath tub. There are seven potential reasons for this (there may be more – tell me please).
If you’ve just had a bath there will be small pools of water at the bottom of the tub. Sometimes cats like to lick up this water. I think this is the domestic cat’s instinct to hunt down sources of water and when they find it they are pleased with themselves and they drink it. It doesn’t matter that you have provided freshwater in a bowl next to their food. There is a potential lesson to be learned. Domestic cats are quite poor drinkers because they’re designed that way, to survive in arid regions. With dry cat food there is a need to drink more water. Placing water in a place other than near their food might encourage them to drink. A different bowl (a clear one) may also help.
There may be a spider in the bath tub. They do tend to find their ways into baths by coming up the drain. Inherent in the domestic cat’s nature is the desire to eat insects as a low-priority prey item. Your cat might be in the bath tub eating a spider or dashing around trying to catch a fly.
A commonsense reason is that cats like to explore places so you might find her there simply nosing around. A cat might be drawn to the bathroom because of the body odour left by their human guardian. Her presence is there in the bathroom and they are drawn to it despite the fact that she’s not physically present.
In my experience, you might find a cat suffering from cystitis urinating in the bath tub. When a cat has cystitis (a bacterial infection of the bladder) they will deposit small amounts of urine around the home because they can’t help themselves. They have an overwhelming desire to pee. Sometimes, out of politeness it seems to me, they pee in the bath tub. It is almost as if they are saying, “I am sorry but I’ve got to pee and I did it in the bath tub because I know you don’t want me to do it on the carpet”.
The texture of the bath tub might interest a cat. It is smoother than the usual substrates upon which a cat normally walks. Inquisitiveness may compel a cat to investigate.
A sixth reason is that the bath tub is a place to hide and find some peace and quiet – a retreat. It may provide a sense of safety although cats generally prefer high spots to achieve this sense of security.
Finally, if the weather is hot, the bath may be cool. It may be cool because (1) it is cast iron and (2) it will have cooled during the night and retained that coolness into the day because of its mass. A cat might like to be there to cool down in hot weather.