It is not unusual for an unused bathroom drain to start smelling after a period of time. The smell coming from an unused tub or drain is usually coming from the P-trap.

A P-trap is the U-shaped portion of pipe underneath the drain. The trap is supposed to keep sewer odors from coming into your bathroom and home.
When a drain is used, the water that collects in the P-trap serves as a seal to keep out the unpleasant sewer odors. If a drain is not used for a prolonged period, the water in the trap eventually evaporates, and the sewer odors are allowed to come into your home. Drain cleaners and household cleaning products may mask the odors temporarily, but they do not stop the evaporation that is the underlying cause of the problem. In most instances, the problem is easily solved by running some water to create a seal in the P-trap. You can do this as often as necessary to control the odor.

If the odor does not improve after running the water, there are a few additional household remedies that you can try. You can try pouring a cup of vinegar down the drain along with a quarter cup of baking soda. Allow the mixture to sit for approximately 15 minutes. Rinse out the drain by running hot water for at least three minutes. Once the P-traps have refilled with water, pour a couple of tablespoons of vegetable oil down the drain. This will serve as a barrier and prevent the water in the P-trap from evaporating as quickly.

If you still have an unpleasant smell after trying all of these steps, you should contact a plumbing expert.