There's a coffee-dispensing machine (similar in concept to a Keurig) in my apartment lobby for use by the tenants. As coffee is a popular morning beverage, occasionally I have to wait behind one or two people. The past couple days, I have had to briefly wait to use it (very minimally, less than a minute), and the person in front of me has turned around when finished and said something like "It's all yours" or "I'm done."
Of course, these statements could be unnecessary; they can just grab their beverage and walk away. After all, you don't yell "I'm done" to the car waiting behind you at the gas pump when you finish filling your tank! But I am not sure how to respond to this. I've been just muttering a "Thank you," but that is also unnecessary (thanks for what? Being done? Or "allowing" me to use the machine?) and I'm afraid that can come across as sarcastic.
How does one "pick up the baton" in the least socially disruptive way?