You are likely to find a sniffer almost anywhere. However, there are some strategic points that a cracker might favor. One of those points is anywhere adjacent to a machine or network that receives many passwords. This is especially so if the targeted machine is the gateway of a network, or a path of data going to or coming from the outside world. This could exponentially expand the cracker's sphere of activity.
Sniffers represent a high level of risk.
In fact, the existence of a sniffer in itself shows a high level of compromise. That is, taking the case study out of the LAN and into the Internet, if your Internet-enabled network has a sniffer, someone has breached your network security. One scenario is that he or she has come from the outside and placed a monitoring device on your network. The other scenario is that one of your own is up to no good. Either way, the situation is grave.