There may be a small % of users hitting the 404 on their smartphones,of which an even miniscule # of users will be in the vicinity. Another alternative is that

There may be a small % of users hitting the 404 on their smartphones,of which an even miniscule # of users will be in the vicinity. Another alternative is that Foursquare could send a push notification for such alerts,embedded in check-in messages for users in the vicinity.