Intrigue.. doubt anyone will know..

Saw a detached house today that looked like some kind of establishment. It was painted grey and had paper scattered around on the windows. Possibly to stop people looking in. Not sure if anything was written on them as there was a sign saying CCTV in operation, we daren't go any closer. This house appeared to be empty. There was also a letter box with the words post here handwritten with arrows. What on Earth is that about?