Drop zones are Bycyklen stations without (active) docking points.
Press the "Return bike" button in the upper right corner of the bike tablet to return a bike in a drop zone. Please note, that the payment continues until you have logged out on the tablet and locked the bike securely using the lock on the rear wheel.
If you want to use a bike from a drop zone, just log in on the screen as usual.
The drop zones are usually marked with striped tape or boards, but if they have been removed, check the Bycyklen station’s status by hovering the mouse over it in this overview by Citymapper.