Quite often info about you being gone is relayed by someone who you see from time to time in your neighborhood (i.e. a neighbor). They know your patterns and or are lucky enough to notice when you are walking out the door with a bunch of suitcases and freshly combed hair after a pre-flight shower. They know if you live alone and or have family members who are going with you.
Or someone you told about your trip told someone else who happens to know where you live. Any number of things. Best bet is to do all the timer stuff with the lights AND the TV...my personal favorite is a radio set to an all-talk radio news station which is near the door and turned up just enough to where it sounds like someone is talking inside. It's not so high as to be obvious of its source, nor its intent. You can't quite make out exactly what it is and is only somewhat obvious if you put your ear right up to the door.
Oh, and don't tell anybody who doesn't have to know that you are going on a trip. If you are gone for a while, have a trusty neighbor grab your mail and newspapers (DO NOT suspend your subscription for a short period of time), and last, but not least have them throw a bunch of their garbage in your cans and recycling in your bins and have them roll it out and back in on the appropriate days. Mailmen, newspaper deliverers, and especially garbage men are good sources of information for thieves as to who is on vacation.