Water stain around pool deck

[shiyang100]

A very dump question.

We closed the pool a few weeks ago. No rain since 3 days ago. I suddenly noticed the water stains around the pool in some areas, as shown in the pic. I am sure no one dump water there. So curious where the water comes from. I am afraid of the leak if the water is from underneath.

 

[pooldv]

My guess is that it is condensation from the cover and/or dew from the fence. The pool is probably quite a bit warmer than the air overnight.