Nice! I bet it stinks! :P There's been several of these hybrids that have caught mice, including a rat in Australia :O.
In addition to growing plants, I design and build RC planes powered by Tesla batteries. Check out my progress at www.chargedplanes.com
It actually doesn't smell much!
I think the sheer volume of the pitcher and liquid in comparison to the small mouse and the fact that it was nearly dissolved covered it well. (There is actually heaps of large digested snails and slugs in the same pitcher)
But then again my plant is growing outdoors out in the open under shade cloth.
As to how it got there, I have seen fleeing mice hop and jump 30cm high and scale double story buildings like ninjas. Poor guy probably went in for a drink ?