There are net curtains Pegasus, see photo 9. You can see to the right of the ruler, on the windowcill.
Better view here
http://www.mccannpjfiles.co.uk/P9/09_VOLUME_IXa_Page_2313.jpg
I agree with Anna that the 'heavy' curtains don't look too heavy ... I would think heavy guage cotton? and the photograph indicated certainly shows nets ... so it could be either or.
I don't think too much reliance can be placed on the crime scene photographs because imo among the first places you would look for a missing child would be under beds and behind curtains so the positioning would be disturbed.
We know that Dr K McCann looked out of the window, perhaps disturbing the curtains in the process.
It was when she pulled the door towards her that it banged shut ... suggests to me that another door was open.
If it was the bedroom patio door that would not have been seen from the stairs to the apartment.
In the couple of minutes taken to check Madeleine's bedroom and cross to the window to look out before searching the apartment, intruders could have quietly closed the glass door and lowered the shutters and made good their escape without being seen.
What is the drop like into the flower garden? Too risky to use the stairs? although sometimes needs must; but exiting from the balcony into the garden, accessing the lane at the back and out into the lane between the two buildings would enable them to stay one step ahead of the alarm and searchers, take them in the direction of the Smith family at about the appropriate time.
The Smiths were walking towards the crime scene ... but apart from that one individual ... they are not recorded as having witnessed any furore or searchers despite the search for Madeleine taking place.