Most movies are comprised of footage that was shot from scratch, but there are times when filmmakers cut a few corners during the development process and recycle scenes from elsewhere, in that hope that nobody will notice.
There are many reasons why a director might choose to dust off old footage and attempt to slip it in under the radar, from time constraints to budgetary concerns, and in most cases this is done subtly enough to go undetected.
You might think this practise is exclusive to indie filmmaking, where funding is tight and FX is created on shoestring budgets, but in reality, some of the biggest blockbuster franchises in Hollywood have turned to recycling to complete their movies.
Some filmmakers have done this for creative purposes, Back to the Future Part II being a notable case, during those scenes where Marty McFly time jumps to revisit the events of the original, though few examples are as blatant as this.
Sometimes movies serve you reused footage and you don't even realise it.