When you check-in to a location on Foursquare but uncheck the "Tell my friends" option, the service will check you in "off the grid," noting that you're somewhere but hiding the location.
Ever checked-in "off the grid" yourself? Would appreciate five minutes of your time to complete this brief survey. All results are anonymous and results will be published on my blog at (http://elapsedtime.blogspot.com/). I will tweet when they're ready (follow me http://twitter.com/hunterwalk) - hopefully by August 7th.
If you have any questions feel free to contact me: email@example.com. Note - i'm doing this survey because of personal interest. Nothing to do w/ my day job.