FATE Core has an entire resource allocation mechanic hidden in a sidebar on page 137 of the kickstarter preview. It's subtle, but I thought I would bring it to your attention. Because that's what the resource in question is; Attention. The sidebar in question is about passive vs. active resistance, and when to use it. And one suggestion it makes is that an NPC who is unaware of a PC should make a passive instead of active resistance. That makes intuitive sense, and I like it. I like it so much, in fact, that I think it can be generalized. In an information soaked world, there's a lot of interest in how we allocate our attention. And I think an interesting FATE mechanic can be found in that concept.
Here's the idea: Active checks are checks your character makes while focusing what they're doing. You roll dice for these checks, and may invoke aspects on these checks. Passive checks are checks you make when your character isn't paying attention. For these, you simply use your skill level and any stunts (or locked-in aspects, if using the stuntless rules) that might apply. Generally speaking, your character is paying attention to whatever they're doing. This means most actions you take will be active checks, and most passive checks you make will be defense checks. Regardless, you may only pay attention to one thing at a time.
Usually an active check is better than a passive one, if you've got an invoke-able aspect on hand. 40% of the time rolling is actually worse than not rolling, but having a single aspect to invoke changes the "worse than normal" range down to roughly 2.4%. Because being able to invoke aspects is the real advantage here, I'd probably want provide my players with slightly more Fate Points when using these rules. On the other hand, this does mean that if you don't have an aspect you can invoke, you may not want to actively defend. I'm OK with that, and wouldn't make anyone take an active defense if they didn't want to. You're giving up the ability to hit the high numbers in exchange for predictable results.
So what does this rule do? The first thing we get out of this are fairly clean surprise rules. If someone takes your character by surprise, then you'll defend against them with only your skill, no aspects or dice to defend you. Since your attacker is making an active check, and probably has an "Ambush!" aspect to invoke, you'll be at a sever disadvantage. It also covers being at a disadvantage when outnumbered. If you can only pay attention to one opponent at a time, then all other opponents get to deal with your passive check. Unless they're a mob, in which case the rules treat them as one entity.
The second thing we get out of this is a good way of handling Notice checks. If you're actively scanning your environment, that's a declared action and you roll. The rest of the time, however, you're not oblivious. You still have a passive notice check equal to your notice skill. You'll automatically see anything in your environment who's difficulty to spot is equal or lower than your notice check. If anything new happens, well, just let players know anything with a difficulty to spot below their notice skill. You'll never tip your players off by calling for a notice check again.
The same thinking applies to knowledge skills like Lore. Any fact pertinent to the situation with a difficulty to know less than the player's Lore skill, they know off the top of their head. Info-dump it to the player and move on. Any fact with a difficulty higher than that, well, they'll have to take the time to think about it.
Whenever adding or quantifying a new rule, you're also creating fertile ground for new stunts that allow you to break those rules. Perhaps a stunt called "appropriately Paranoid" allows you to always make active notice checks. Another, perhaps "Fighting stance", would let you actively defend against an additional attacker. But since I prefer stuntless rules, I like to add the following to general invocations. You may invoke an aspect on a character to immediately redirect that character's attention. So you would invoke an "appropriately paranoid" aspect to shift your attention immediately to the notice check regardless of what you were doing. Or you might invoke an enemy guard's "Tired and Bored" aspect to direct their attention away from the notice check against you.
Another possibility is that you might "actively" use passive checks when taking certain kinds of actions. Certain kinds of training seep into your unconscious mind. Someone with good proprioception is less likely to trip and fall, and people trained in moving quietly do so even when not paying attention to it. This kind of unconscious training can be represented by passively using these skills anytime you move. So someone with the stealth skill, unless they say otherwise, is always moving with some amount of stealth. And someone with a high athletics skill will automatically overcome obstacles below their skill level when taking their free move. In fact, I'm tempted to generalize this to all skills. A rule like "A character is always considered to be passively using any skill they have ranks in" would have far reaching and interesting effects on gameplay. I haven't figured all of them out, but its something I'm certainly going to focus on as I work on my FATE hacks.