My suspicion, which could be very very wrong, is this -
If you are not staying at a Disney resort, and you park in a monorail resort "to have breakfast" or "just to look around" they will be sticklers with the 3 hour reservation window.
If you ARE staying at a Disney resort and do the same, Security may or may not be strict about the 3 hour window. Because after all, you have free parking in the TTC lot anyway if you so choose, so it's not like they're losing money by letting you park anywhere else on property. But the busier the season, the more booked the resort is, the more they'll be apt to search the lot for cars that technically don't belong.
(The next I say hypothetically, I've never done it, and I wouldn't recommend you do it)
I wonder if they would be so strict at Wilderness Lodge or Fort Wilderness. First of all, it's not particularly convenient the way the monorail lots are, you certainly won't be saving much time if any. But if an offsite guest were to, say, make reservations for a super early breakfast at Whispering Canyon Cafe or Trails' End, parked not so close to the front, actually ate breakfast, then took the ferry over, if they'd give you guff. I suppose they might because you are, indeed, taking a parking fee out of their wallet and keeping it in yours. But then again, that breakfast you ate, you might not have eaten there, so they lose that money.
Oh well, I'm DVC so hopefully it'll never be an issue with me. AWWWWWWWW, SNAP!