The Spartan may be inside of an iberic set of plate armor, but he notices his rutsack moving -- hence why he grabs a dagger as soon as he opens it... to see nothing else inside, save for the watering can.

Which happens to be moving.

Shaking.

The knight picks it up to stare at it, keeping a weapon ready. The water may be polluted for all he knows...

... So expecting a rat, he taps it.