GRE packets

Moreover, if IPSEC is in use, you can't even sniff.

With IPSEC you infer things from traffic analysis. Or if you don't
feel like doing grevious harm to IPSEC, you block the traffic and wait
to see who complains. Many telephone companies do this type of blocking
to control fraud on international calls. Your calling card will suddenly
just stop working. You'll call up your communications provider, and while
usually acting totally clueless, they'll re-enable your calling card.

Most of the time, the spikes in traffic are fairly obvious. People
share the 'secret' with their friends and like hogs at the trough they
can never use it just a little.