I don't have the side skirts (Mates cupra does)but they do have a retaining strap on the inner.
Are people not replacing the covers correctly after removal or am I missing something.
Its possible that where the strap fastens on the inner that this piece could get damaged when removing the cover hence loosing it on a journey as the strap is not fastened in place properly. No ?

the strap you talk about is fragile and snaps off easily , about as much use as a chocolate tea pot.
 
Use clear silicon

Had the same problem on my leon cupra tdi. Thank christ for the attachment cable thats on them already would have lost it for good ! I just put clear bathroom sealing stuff round the edges. Holds it tight and water tight ! Also just a good pull and they off, quick and clean to put them back on too, just peel off the old stuff and on with the new !
 
haha this is how i stuck mine on with clear silicone. Like you say come off quite easily when you need them to and silicone peels off quite easily.
 
Can anyone point me in the direction of a good inferi black paint? Lost one today, sodding things only had the car two days!!