Complete the detective's highly professional stakeout instructions.

When the suspect finally arrives at the warehouse, we ___ in the unmarked van across the street, quietly eating donuts.

The correct answer is will be waiting.

We use the future continuous to describe a longer background action that will already be in progress when a shorter future event (the suspect arriving) happens.

Notice how the shorter event uses the present simple ("arrives") because it's in a time clause starting with "when".