Who says looters need to destroy everything in their path? Any tomb raider worth their salt should be able to bypass threats and get to the goods instead of brawling it out with everyone along the way.
I don't know what kind of creature it is, but it's entirely possible that the guardian of the previously raided vault was put to sleep, controlled, or in some other fashion incapacitated without being destroyed or "defeated".
Alternatively, it's ancient magic lost to time that animated the guardian. Perhaps after being defeated the magic pulled it back together, to guard an empty prize.
Basically....there could be in any number of reasons to justify the way it is. Don't sweat it.
Yeah, I'm probably overthinking it. It is a Construct, though so it's immune to many things.
I guess I'll go with the latter of what you said, that it was reconstructed over time.
Well, don't forget the existence of the Impossible Bloodline Sorcerer.
I mean sure, that's a very specific ancient thief, but still a possibility.
Besides, the magic that overcame the guardian the first time could have been nearly as ancient and simply work very differently than the magic we know in setting at this time.
To be honest, you don't have to explain it to your players in my opinion beyond "it's ancient magic".