Hannah7 - never been outside the US for anything close to 6 months. My longest trip abroad is 8 days, although I do have many short term trips to Mexico, varying in length from a few hours to about 5 days. The duration of my trip has never been a problem upon re-entering, but I bring documentation with me that proves my current ties to the US. Typically, I bring copies of:
1) Mortgage statement, with address matching my driver's license
2) Last year's tax returns
3) Copies of recent pay stubs from my employer
Yes, this can be a bit of a pain, but it's worth it to avoid any hassles at the border. Since I have nothing to hide, I will gladly offer it up if it becomes an issue. I bring the documentation regardless of where I am going and irrespective of whether I fly, drive or walk across the border.
Just as a sidebar, when driving to Mexico, the US has license plate scanners in place that record the date and time of the vehicle's exit from the country. Upon your return, they again scan the plate and know exactly how long the vehicle has been out of the country. When you fly, there are passenger manifests, and possibly passport stamps as well as passport and GC swipes that record your entry electronically (airlines generally swipe your passport when you check in, thus creating a record of your departure from the US.) When you walk, there is not exit control - you simply walk through a large metal turnstile. There are, however, closed circuit cameras that record the exit of all pedestrians exiting the US. To my knowledge, facial recognition software is still a long way from perfect, but I imagine that within a few more years it will be used, particularly in trying to match up faces with law enforcement "most wanted" lists. However, as a GC holder, the government has both your picture and your fingerprint on file so there is nothing stopping them from using that technology to get a sense of who is departing and re-entering. Upon re-entry into the US at a pedestrian crossing, video and audio recording occurs at the border (there are signs up at border crossings making travelers aware of this fact.)
If you venture too far south of the border in Mexico, or stay in a border region for more than 72 hours then technically, as a foreigner, you must report to Mexican immigration and get a passport stamp in your passport. If you don't do this, you are breaking the law in Mexico.
Generally, passports are not stamped at land or sea crossings, but again both passports and GCs can be swiped electronically.
Hope that this helps you you with any information you are trying to glean. By telling you this information, I am not in any way encouraging you (or anybody else) to try and break the rules and pull a fast one on the government. On the contrary, I am trying to talk you out of doing something that breaks the rules that you will later regret. As you see from the above, the system is designed to prevent you from trying to obscure your absences. As well, if you ever apply for citizenship, they can and do ask for employment, tax and residency records to verify that you were, in fact, residing in the US.
As a final note, remember to record all exits from and re-entries to the US. This will be useful if you ever apply for citizenship. In my case, I didn't do this for all my trips of > 24 hours, so I was forced to make an estimate in addition to trips that I had a record of.
