Why do white translucent spots appear in night photos when using flash?

These spots are usually caused by light from the flash reflecting or scattering very close to the lens.

From the answers, the most likely causes are:

• dust, mist, or water droplets in the air catching the flash
• smudges or fingerprints on the front of the lens
• possibly a dirty lens surface in general

Because the reflected light is very out of focus, it shows up as soft discs rather than sharp dots. They stand out much more in night photos because the background is dark.

To reduce it:

• clean the front element carefully
• avoid shooting through mist, drizzle, dust, or breath condensation when using flash
• if possible, move the flash farther from the lens axis or use an external flash to reduce straight-back reflections

So the effect is not usually a sensor problem; it’s most often near-lens reflections made visible by flash in dark conditions.