I just read an article that describes a burglar as posing as a buyer at open houses as his modus operandi. Houses in high dollar areas tend to be larger and with more than one visitor at a time, a single agent can lose track of who is where. This guy allegedly focused on jewelry which is small enough to conceal easily.
If you're doing an open house, be on your guard. If you are attending an open house, please understand that there are reasons you are asked to sign in, beyond just getting your name and contact information.
Here is a link to an article about this topic.