The small hole near the top of a bathroom sink is an overflow hole designed as a backup drainage path.It prevents water from immediately spilling over the basin edge if the faucet is left running or the drain is blocked.
When water reaches the opening, it flows through a hidden internal passage that connects directly to the lower drain pipe.

Because its internal channel is narrow, the overflow hole cannot handle full faucet volume indefinitely and only buys extra time.Not all sinks feature this hole; many kitchen and vessel sinks omit it due to their depth, material, or design preference.
Over time, the damp, hidden channel collects soap scum, toothpaste, and bacteria, which can cause unpleasant sink odors.

Smells often persist after normal cleaning because people routinely forget to scrub or rinse inside the small overflow opening.
Using a small bottle brush, pipe cleaner, or warm water flush effectively clears out trapped residue and restores freshness.

Maintaining a clear main drain ensures that any emergency overflow water actually has an open path to escape.Ultimately, this subtle design feature serves as a valuable, built-in safety measure protecting homes from accidental water damage.