It seems the latest update (the one for mini-game rewards), has added runecrafting xp to pyramid plunder. The past few day's I've been hitting plunder in an effort to get the new "ibis" outfit. At the end of every day I review my xp stat's, and the past few days I've gained runecrafting xp, yet I haven't gone near a runecrafting alter.
I discovered that the new "Sarcophagus" they added to the final room in plunder requires 75 runecrafting to open, and gives runecrafting xp upon opening it. I searched the RS knowledge base, and the update patch notes, yet I found no mention of it. It is however displayed in game through the stat's tab when you do a "view" of the runecrafting skill and look under "mini-games".

So that explains my mysterious runecrafting xp, (I was racking my brain trying to remember what I did to get it). I don't know exactly how much xp it gives per Sarcophagus, I'll check it out the next time I do plunder. I do know I gained around 7500 runecrafting xp yesterday in runecrafting with maybe a few dozen plunder runs.
While I will always take free xp, especially in runecrafting, I find it hard to understand what runecrafting has to do with thieving or pyramid plunder. Am I missing something here?