Puddle In Back Seat?

I have a 2003 Dodge Neon. From time to time, an odd puddle of liquid will be in the back seat, behind the driver's seat, down in the footwell. Several years ago I had my service station look at it. They pulled up the rug to see if there was something leaking and couldn't find anything.   It seems to be independent of the weather. It will happen even if it hasn't rained for weeks, and I haven't driven through any water.   The best the repair shop was able to offer me was that maybe the seals along the back door are loose and water gets in somehow, but that doesn't make any sense to me when it happens during dry weather.   Any ideas what this could be? It has no smell or color.