Care of Exped Mats

Subsequent to feedback on this forum I purchsed 2 exped Synmat 7's from Bivouac in NZ. I decided against the Downmat as I felt that I didn't need the extra insulation (yet) but most importantly I was worried about the potential for moisture to cause the down to deterioate. I took one of the mats for its inaugral outing a few weekends ago and was horrified to find moisture in the non-valve end of the mat after using it. I inflated it with a Schnozzel. It was raining and the tent floor was damp but not sopping. I'm curious as to whether I've committed a basic error in using the mat and secondly the optimum way of drying and storying a mat where moisture has entered. In the case of my Thermarests I've just stored them with the valves open however the Synmats don't sit like the Thermarests when the valve is open.