The library

Rain droplets

Was it raining , or raining even slightly when you took the photo?

The photo below was taken just as it started to rain. I could see the droplet caught in the camera flash as it went off. I have seen many pictures like this one described as orbs.

Particularly if you are using a flash, light may bounce back off the rain droplet and give this orb type appearance.

In this example we can tell this "orb" is a rain droplet by the brightness and sheer amount of light reflected back to the camera.