After logging out, I noticed that I was missing several thousand (about eight thousand) coins were missing. Later I check my inventory and I see that there is a tundra background. This puzzles me, I don't think that I bought this, but is there anything to confirm that I did?

Did you encounter a magic shop while exploring? It costs around that price to buy tundra background from the shop.

No, I am saving up for more stable space, so I didn't buy anything that costly. How much silver could I auction the backround for?

According to the logs, you didn't buy anything from the Magic Shop or on auctions on May 26th. It was something else. Here's the full list of possible silver coins expenses:

• Daily pet food (18 per pet if you don't have enough food in your inventory)
• Genetics research (100 per gene)
• Hiring catchers and sending them to expeditions
• Placing bids on pet and item auctions
• Buying pets and items through private offers
• Accepting breeding offer from another player's pet
• Losing a battle with bid on Arena or in private challenge
• Buying items from the Magic Shop on islands
• Using Forest Alchemist, Dead Tree Altar, Copper Idols and Shaman Hut on islands
• Training a pet. Please remember that silver is not refunded when you cancel the training session.
• Crafting items and constructing buildings
• Using custom skins and apparels with a silver fee

Oh well, better start saving up again, thanks for the replies!