Liquid detected on iPhone without water exposure

Dust and lint in the charging port can result in this message being displayed.

Carefully examine the port with a flashlight, and wooden or plastic toothpick to make sure that the port is clear.

If the message continues to appear, make an appointment at the Apple Store or take the phone to an authorized Apple service shop.

Genius Bar Reservation and Apple Support Options - Apple

FYI, there is moisture everywhere… even in the air. Rapid temperature changes can cause the moisture in the air to condense out into liquid water.

You may not have submerged it or spilled anything on it, but that doesn’t mean it was not exposed to water.