All the following are possible, but listed in order from most to least idiomatic.
The moment we see the police following us we will start to run away. The moment we see (that) the police are following us we will start to run away. The moment we see (that) the police have been following us we will start to run away. The moment we see the police foll
That next-to-last version is what I'm most interested in. I realize this sounds awkward, and I wouldn't say that sentence myself, but as you say, it might still be 'correct'. Of course, it's another of those test questions... This time I quite agree with the key, which gave SEE and (ARE) FOLLOWING (your preferred choices), but language is not mathematics, so I'm curious to know what is possi