A home security system monitored by a security company is supposed to increase your safety and protect your property—but scammers and burglars have worked up some ways to turn these systems to their advantage and victimize home owners who have them.

The scam works like this—you receive a call from what you think is your security company saying that there’s a problem with your system that must be fixed…or that your system is due for a free upgrade. Soon a technician appears at your door. This technician might have a shirt or an ID tag that identifies him/her as an employee of the security company. (In some versions, there is no initial phone call—a technician simply arrives unexpectedly. A legitimate home security company will not do this.)

But this technician actually is a criminal who learned which security company you use from the sign posted on your lawn or in your window. If you give him access to your security system, he might case your home to see if it is worth burglarizing and then disable your security system (or get you to divulge your security code) so that he can safely break in later. He also might say that he needs to schedule a follow-up visit, then try to get you to say when you will be unavailable so that he knows when it is safe to break in.

Variation: Rather than burglarize your home, the fake security technician might ask you to sign some ­paperwork that he says is routine but that actually locks you into an expensive, long-term contract with a different and untrustworthy provider.

What to do: If someone calls or shows up at your home claiming to represent your security system company, call the company using a phone number that you know to be authentic to confirm this person’s identity before letting him in. If you’re told this person is not with the company, refuse him entry and, if possible, jot down his license plate number and give this to the police.

If possible, swap lawn signs with a friend who uses a different security company—that way, if a scammer tries to pull this on you by phone or with a surprise visit, he will claim to work for the wrong company, immediately exposing his ruse.