Where does the dirt lie in and around your home?

I thought my house and garden were quite clean and tidy, always a few corners needing attention but the overall standards seemed fine to me. I've just finished watering some plant pots on the back doorstep using the hose, and absentmindedly ran the hose over the plastic tufted door mat.................and was truly shocked by the amount of dirt that came out of it. It didn't even look dirty. I usually turn it over and bash it against the step every week or so but haven't washed it before now as it looked quite clean. I've hosed it for 20 minutes full blast and there is still dirt washing out but I've had to leave it for now.

Has anyone else had a shock to discover something they thought was quite clean but in fact was filthy?