The metal detectors are a more recent addition. I do not know how more recent, since it's been awhile since I've been to the parks before I recently got an annual pass and had been going. Before it was merely a warning, but now they strictly enforce you using lockers if the ride goes upside down. Good thing pcstang for pointing out the Hulk, for I forgot that it would be the same for the Hulk, since the new track I'm sure will go upside down like the previous did. I didn't think of the Hulk since right now it's currently being refurbished.
I bought a small fanny pack (with a 3 prong clasp) in which to keep my sunglasses, cell phone, and money. I should be okay to wear that on all rides except RRR and Dragon Challenge, right? Trying to avoid locker rental when possible!
To be safe, I would recommend before going onto any ride (that has a sign about "storing loose articles" or bags, because not all rides have these restrictions) to ask the person outside the ride if your fanny pack is
required (import to use the word required, not recommended) to be put into a locker, or to just save the trouble all together and automatically put the fanny pack into a locker the minute you see one of those signs. Also, have your fanny pack out and proud that way (in case you do not see a sign) an employee might see your bag and say something about it before you go any further. I recommend all this, because that way you do not go through the whole line, or half way through for an employee to go "You need to put that in a locker before going onto the ride" and having to walk all the way back to the lockers. As that happened to me before.
I say all this, because I would imagine fanny packs could potentially get in the way of lap bars and other means of safety that is provided on rides. That and the fact that when I looked up the question myself I got mixed messages of people saying they were good on some rides, but on others they are a definitely a no-no. It depends on the ride, depends on if employees notice, and sometimes depends on the employee working whether they let it slide, or strictly enforce it.
Conclusion, you're going to be using a locker whether it's once, twice, or several times whether you have a bag, or your pcokets. Recommendation is just to accept it, go onto the rides which you know you'll more than likely have to use them, and go there early to beat the locker crowd. As someone else said, at least if it's in a fanny pack (or bag) all you have to do is throw it into the locker, just grab it in one swipe once you open it again, and know you'll have everything. And at least it's free during your ride.
EDIT: I've also read somewhere that some people (especially on the Harry Potter rides) use Child Swap as a "Bag / Articles Swap" where people stay behind and hold things for other members of their group, or family while the other people ride and then take their place, as Ollivander wands, for example, do not quite fit into the lockers at Forbidden Journey from what I've heard.