When the "relics" are knocked off the wagon driven by the two drunk men, and fall into the bog...is this the earliest example of this plot device? Eight Legged Freaks and some zombie movies have also used it. It's a bit of a staple, but this movie is the oldest one I know to do so.
Is there an earlier one that I haven't seen or am overlooking?
It does seem to be kind of late in the genre to use it, but most of my searches and viewings of earlier movies of the type have nothing like that. By the 80s, every other horror movie has some idiot opening something, dropping something, or such...and in stories there were drunken idiots who would stumble into graveyards or end up asleep in crypts or such, but the old "barrel into the bog and let's not say much" trope is fairly specific.
Don't forget the original "Frankenstein" from 1931 has Fritz dropping the normal brain and then stealing the criminal one, and failing to mention this little episode to Dr. Frankenstein. So that seems to predate this by almost 30 years.
Also, the drunk men did say something. The next day, there is a scene where the swamp is being checked by police and others looking for the lost cargo. So the idea that 'let's not say much' isn't accurate. They clearly did say something and thus a search was conducted in the swamp (unlike Fritz, who never said anything to the doctor).
What I don't understand is the actual point of this part of the story. What purpose is served by having the mummy sink into the swamp? The evil Egyptian guy was still able to retrieve/resurrect it after the fact. Whether the drunk guys lost the cargo or not, the mummy still wound up with the Egyptian and still wreaked havoc. Unlike the Frankentein story, where the damaged brain made a tremendous difference to the development of the plot, this one seems to have no purpose at all, other than the cool effect of the mummy rising from the depths of the swamp.
It served two purposes. One being that it gave them a way to add more clues for the inspector to dig up. And the second one being that otherwise it would have made no sense for the mummy to bring the girl to the swamp at the end... well that still kinda makes no sense but whatever =P