From the information here, I can't find anything that guarantees that this Giratina is hacked. But, I believe it is very likely.
First: let's assume it's not hacked. As steamcrusader mentioned, the "traveled through time and space" means that it was caught in Gen 4, and there weren't any shiny-locked legendaries until Gen 5. That means that, yes, soft-resetting is indeed possible (more on that later). That also means it could have been transferred to ORAS to receive the Hoenn champion ribbon, and then transferred to S/M to receive the Alolan ribbon.
So, yes, this Giratina is theoretically possible, but here's why I think it's probably hacked.
As steamcrusader said, "alert to sounds" corresponds to its IVs. This means that its highest stat (or one of its highest, in the case of a tie) is speed, and that the ones place of its speed IV is either 1 or 6. So its speed is either 1, 6, 11, 16, 21, 26, or 31. And while I can't say for sure, I'd be willing to bet that its probably a perfect 31.
So, if we take its shininess, Timid nature, and assume that it has 31 Spe IVs, here's the chance of encountering this Giratina per reset:
Shininess (in Gen 4): 1/8192
Timid Nature: 1/2 (with synchronize) or 1/25 (without)
Perfect Spe: 1/32 (0 included)
If this person is smart and used Synchronize to optimize the chance for the nature, the chance of encountering a Giratina like this per reset is 1/524,544.
Sorry I bored you with my math. Long story short, by the time you would have found this thing, you probably would have found more than 50 other shinies. And at that point, are you really just going to put it on the GTS? Yes, technically, it's not impossible, but I honestly have no doubt that it's hacked.