..... (much cut)...
Other things from movies/books that might work:
- repeated wall markings from previous adventurers like AS (Arne Saknussemm) from Verne's Journey to the Centre of the Earth.
- increase of rat swarms fleeing from something (can be small enough to not do any damage and rush straight past the party) from just about any number of disaster movies.
- foreshadowing of stuff from other sections/plot lines (Babylon5)
- change the colour saturation/hues of the landscape as evil ebbs and flows, or temperature changes up/down from the lava flow felt in the non-lava flow areas of the dungeon (most VR type movies have something like this happening, including parts of the Matrix trilogy).
You don't have to act on any of these - all they do is indicate that change is occurring, and the party's actions might well be the small events that precipitate other things (Jurassic Park - read the book, not the poor interpretation that was the movie).
enough rambling - I'll get back in my box now