I follow the Bernalwood blog; it’s well-written and entertaining. Recently, they’ve been talking about blue masking tape in potential criminal activity.
The way it works is this: The criminals put a small piece of blue tape on the door (or, I suppose, the garage door) of a house – or many houses on a street. Then they come by at night to see if the tape’s been disturbed. If it isn’t removed for a number of days, they can guess that the house is unoccupied and therefore safe to break into.
Apparently, the Bernal area has had a rash of these blue-tape tags. There’s no report of subsequent break-ins, at least on the blog. But many of the neighbors there have been removing tape-tags they see.
Why does it have to be blue? I don’t know. I’d guess white masking tape would work as well.
I haven’t seen or heard of anything like this from any other source, but if you’re aware of it, let us know?
FOREST KNOLLS NEIGHBORHOOD 
There are a lot of neighborhoods – ours included – where people don’t use their front doors every day. That’s because they come and go through their garages. So that is a dubious method of determining whether or not people are home. A person could also be sick for a week and not go out, even if they do use their front doors.
Good to know, though! Hopefully they’ll be caught through their faulty logic.