If your blockchain and wallet are synced and it is still missing without a transaction visible, this can have happened:
Someone with access to your mnemonics installed a Chia Light Wallet Beta or a Chgia v1.3 Beta client, entered your mnemonics and transferred your XCH to another wallet.
Because the v1.2.11 wallet is not compatible to the v1.3 wallet (also the light wallet beta) you can not see that transaction in your client.
You can check by entering your wallet address in a block explorer such as xchscan.com where that transaction should be visible.
I tried this myself when figuring out a similar situation.
https://chiaforum.com/t/standalone-light-wallet-simply-lovely-but-hackers-can-go-into-stealth-mode/15443