There are several reasons why ants might be found in your bathroom, even if there is no food present. One possibility is that the ants are coming in from outside through cracks or holes in the foundation or walls. Another possibility is that there is a hidden food source in the bathroom, such as crumbs in a drawer or cabinet, or even a leaky pipe that is attracting ants. If you have ruled out these possibilities, it is possible that the ants are actually coming up through the drain in the bathroom sink or tub. This is most likely to happen if there is a colony of ants living in the soil beneath your bathroom. If you suspect this is the case, you can try pouring a pot of boiling water down the drain to kill the ants.