Cold spots under patio slabs?

[James South]

So we had a patio laid about a month ago and it's looking really nice. Twice this week I've spotted on two or three slabs (of a total of 35-40sqm) these little marks. Pretty much the same patern on both days.
They go as the day progresses and only visible when that part of the patio is in the shade.
Is that just cold spots? it was deffinately laid on a full bed of mortor so just wondering if that's perhaps small sections that's not taken as well? Surrounding slabs appear ok.

 

[jj4091]

Dew on some slabs that are slightly more porous than others?

 

[James South]

Possibly. I've just been out and noticed it's kind of drying but leaving a white mark. Could be efflorescence (from my googling!)?

 

[James South]

More of theses spots this morning. It was colder last night but again only on our front patio, not the back (which gets a lot more direct sunlight). Could it just be the end of the curing of the mortor as its only been down about 4 weeks? Its random slabs but more of them today and no real pattern.