I am amazed at how often clients ask about home security signs substituting for a security system. Some think that just putting a sign in their yard saying "Property Protected by _____" will stop a burglar from entering their home. In actuality, these signs can be effective but in the majority of cases, they are not. A security sign or sticker does not automatically flip a switch saying, "RUN!" These signs are so common that a burglar may still try to enter the property. They will take their chances that the system is deactivated or non-existent especially when they see things like an open window or door.
Now putting a security sign or sticker is definitely an inexpensive alternative to a security system but it all comes down to the burglar. Results will vary on a case by case basis as some burglars will think before they act. If you have a sign that says you have a security system, there is a good chance that the burglar will move on to the house next door that does not have any indication of protection. However, you never know how dedicated or ignorant a would-be thief can be. The last thing you want is to come home and find a breach of your home.
You should think of a security sign as an extra layer of protection on top of a security system. Many people also think that they are fully protected just because they have a security system. In their mind, they do not need to pay for and install the signs (which most companies will provide free of charge for advertising purposes if you use them). What they fail to realize is that they leave a huge vulnerability in their protection. Even with the system in place, a burglar may still attempt to enter the property because they do not see any indication of a security system.
My suggestion is a combination of a monitored home security company with security signs and stickers posted throughout the property. I would recommend placing them on windows, garage doors, mailboxes and fences. In addition, you should mention to friends and neighbors that you have a home security system as you never know who might attempt to burglarize your home. With this combination, you have the best chance of preventing a burglary or property theft.