Minecraft – this block I keep seeing in Tekkit with Sips and Sjin

As Ben Blank said, the only thing that will spontaneously cause an explosion, if you haven't made any TNT, is a creeper that got into your house. I see three possible directions for the creeper to enter from:

1. The caves

   Most likely, either you have an insufficiently lit patch of floor in the connected caves (walls and ceilings don't matter), or there is a dark branch that you haven't found. I strongly recommend always installing doors at connections between caves and living space.

2. The front door

   The damage you described seems to be focused around the doorward side of you. You say you removed "the bricks leading up to the door", but if there is still a one-block-jump path it is possible a creeper got in anyway. Next time, don't worry about removing the path, just close the door! If you have a wooden door, zombies can break them - this will cause a loud sound of someone punching some wood.

3. The ceiling

   You haven't said anything about your ceiling design. Is it possible that your ceiling had a hole in it through which a creeper entered? There are [minimal spoilers] ways for dirt/grass blocks to go away, so if you had a dirt roof perhaps a creeper fell in.

Miscellany

To my surprise, it appeared that nearly all my objects were still there. Most were sitting in the same spot that their chest was in. As best I can tell, I was able to recover nearly everything, with the exception of the signs and 2 of the 5 chest blocks.

Chests drop their contents when destroyed, as you saw. If there had been two explosions, then the second one would have destroyed all of your resources, as they have almost no “health”.

They also tend to leave big craters, and there seemed to be too little structural damage in this case.

The size of an explosion crater depends on the strength of the materials. If you meet a creeper outside, then it will leave a large hole in the dirt, but the same explosion will do much less damage to stone.

Oddly, the door and torches outside were gone, but the structure itself was untouched save for two holes in the floor (marked "Gone" in the above picture).

Wooden objects are generally weaker against explosions, and torches have no blast resistance at all.

I'd say that the most major difference is the addition of the Thaumcraft and Mystcraft mods in Technic. As others have said, Tekkit is for use in a multiplayer setting, while Technic is for SSP only.

The major mods in both are IndustrialCraft2 (adds electricity and industrial machines), Equivalent Exchange 2 (adds in alchemy using the Philosopher's Stone, and several highly powerful end-game tools like the Void Ring, Ring of Arcana, etc), Redpower 2 (adds in redstone wiring and machines), and Buildcraft (adds in some automated mining machines as well as transport tubes and controls for that).

In addition to those core mods shared by both packs, Technic adds several features that are best in singleplayer. Major differences are:

• Thaumcraft / Thaumcraft 2: Thaumcraft is a magic and arcana inspired mod for Minecraft. It features aura and vis (pronounced "vees"), which introduces a new mechanic for enchanting and crafting unique items. Thaumcraft also introduces the "Taint" - if you've ever played Terraria, the Corruption from that game is similar to the Taint in Thaumcraft.
• MystCraft: MystCraft is a mod for Minecraft inspired by the Myst series of video games. It allows the player to craft different types of books which allow for teleportation between different "ages", which are basically new worlds. As you get more advanced, you can write your own books, which lets you customize the ages to your liking. For instance, I wrote an age with the "Hell" modifier, which spawned me in a world with a ton of Eldritch Obelisks (artifacts of great power in Thaumcraft), which helped me to get ahead on my research.

A few other minor changes worth mentioning:

• Somnia: Simulates the world while you're sleeping. So, unlike vanilla Minecraft, when you sleep, stuff smelts, crops grow, etc.
• Mo' Creatures: Basically what it says on the tin. There's a ton more creatures in Technic than there are in regular Minecraft.
• Treecapitator: Handy little mod so that, if you chop the bottom of a tree out with an ax, all the wood in the tree falls down at once (kind of like real life, eh?).