There are yards and there are yards. If they spent twenty grand in landscaping you probably won't get a green light. On the other hand, there probably isn't much there to find, either.
With the "I Find Lost Items" business card, knock on the door and ask if they have lost anything. A ring while gardening? A birthday silver dollar? Or? It's a smooth transition from "Can I help you?" to "Can I look?"
"Well, if you don't remember losing anything would it be OK if I checked to see if anyone else did? I'll splint anything I find with you 50-50."