My Bitcoin was Taken

I have a hardware wallet with iCoin Technology. The Bought from the website, everything was sealed. The device itself cannot connect to the Internet but you use the app on your phone to check balances and create QR codes for public addresses. When I first opened the device I printed seed phrase and put it somewhere in the house that's not visible and told no one. No one has broken in, only family has been over a few times but again no one knows where the phrase is. I've had the wallet for almost a year. Never had a problem with it. Today I was checking my address to send more to the wallet and it showed zero. When I checked the transaction history it showed everything was taken two days ago on the 16th. How can this happen if someone doesn't have the seed phrase?