I would be weary of telling them, it depends how 'friendly' you are with them. Either way it may be good to find an excuse to move their bin for an excuse of finding their key and then telling them you found it and they'd better change place, and you yourself do not leave keys under mat, plant pot, stone, bin etc
Remind me of something similar happening to us. When we were looking for our first home we saw one we liked in an Estate Agent and when asked for a viewing. After a couple of weeks still looking for a house (when the market was moving very fast) and reminded the Estate Agent about that house...he said that the owner was a single vet working long hours and never home and other people were interested in viewing but they could never get hold of him.
So we asked the address so we could at least check out the neighborhood. We got there, we rang the door bell, no answer, it looked as if no-one was home so we peeked through the window, then the letterbox and as I touched the flap felt the door move... it has not been closed properly!
We shouted Helloooo, Anyone homeeee, but no reply... foolish I know but seemed exciting at the time, we gave ourselves a tour of the house.
Got back at the Estate Agent the day after and put an offer in (we did buy it at the end), he thought we were mad buying without viewing the house lol....