Hiding messages in plain sight…
Anons should know this.
Forums are very fun places to leave messages. Fan forums are really good. High member traffic preferable in many cases. Alts can be used, much like they can here. However, it can be more profound as accounts can be made to act according to different personalities. Messages can be hidden inside of apparent arguments intended for another party.
The choice of forum is also important. No one thinks twice about talk of plunder on a pirate board. A show that is popular, long-running, and with themes that allow a wide range of topics to be embedded into metaphor works very well. A conversation can appear as a pedantic argument among a community… But it is actually settling some manner of finances or who is to go attend school in another country.
Forum features can be used, as well. When I change my avatar, it changes everywhere. Most forums do not save a publicly accessible record of my profile, and some page archiving methods will still point at the server function to call my avatar, rather than save the image that was there a month ago.
Same with forum signatures.
Status and other settings can be preserved as a history… Allowing for the passing of a message that is less time sensitive, but not on the surface for any forum lurker to trip over.
Forum profile pages and pictures, custom settings… Everything can be geared toward multiple levels of communication that are perhaps "neat" to the average bystander, but contain a great deal of information for those who know what they are looking at.
User names can also be changed on some boards. While many have a long record of this, it means that older conversations can be masked by user name changes that slip beneath the notice of the average user.