Are bitcoin testnet addresses taxed? Here is a testnet address I've been experimenting with,

Here is some test BTC leaving the address,

I didn't initiate the transfer though, is this a feature of the testnet to keep the faucets stocked perhaps?

My wallet is provided by TP's bitcoin faucet from here.

Thanks in advance :)
No, even in Testnet, there is no legitimate reason for coins to be leaving your address without you initiating it.

It's possible you somehow sent the transaction by mistake, or that someone else (perhaps a virus or Trojan) gained access to your private keys.