In a recent warm-up round at Trophy Lakes, I just cleared the lake---always questionable with my arm---and started to walk around, when here came a retriever with my disc, dutifully returning it to me. (With, I might add, some modifications to the plastic).
On my private course, my neighbor's dog has recently taken to snatching discs. He's restrained during tournaments, but there's always the chance he'll intervene....and you don't want your next throw from whereever he's likely to take it.
On a steep slope, I've seen a disc at rest be hit by another disc and slide 20 feet away from the basket.
In a previous tournament at Trophy Lakes, I landing a foot or two beyond the OB lake. A waterski boat passed, and it's wake lept up on the shore and drug my disc into the lake.
"At rest" is generally a good provision.
There is an exception for disc caught in a tree, even if it came to rest, if it falls before you get to the spot.
There probably should be an except for a disc at rest in the basket (wedgie) falling out. Of course, what if you ace, and are resting in the chains, and someone else's disc hits the basket and knocks you clear out? Or a really big gust of wind?