Firstly all images have noise. The reason it is more evident at higher ISO is simply that all high ISO does is multiply the signal level, which includes noise. Because processing to a visual form requires a no-linear transform, the darker part amplified become more apparent than they would be unamplified.

It's like you're receiving a radio signal. It will always have noise. If you amplify a weak signal the noise is also amplified.

Also be aware that even in perfect conditions the air is not clean and without distortions and dust. So, for example, the mythical "clear blue sky" is not.

Note that fine grain is left over from low level noise reduction processing. The processing aims to remove colour noise first and generally leaves luminance noise alone. Luminance noise give that grainy appearance. This is done because the eye is more sensitive to colour noise than luminance noise and because removing luminance noise will normally also remove low level detail. It's a balance.

Lastly note that sharpening done in camera will typically exaggerate grain. This is unavoidable using in-camera sharpening, but selective sharpening done in an editor can avoid it.