Here's what I think I'll use from now on. Roll 1d6 once every time:
- An hour passes,
- The party stops moving,
- There's a loud noise in the same area,
- The party does something that can attract a specific kind of nearby monster.
On 6+, a monster shows up. Roll on appropriate table, unless the action that triggered the roll is only detectable by a specific kind of monster: mind-magic or psi near psionic creatures, for example. Some things may adjust the roll (maybe torches should add +1, candles and burning oil are normal, dousing lights and removing boots and metal armor subtracts 1.)
If there's an extended loud noise (fight lasts 3+ rounds, for example,) the dungeon is on alert: roll every turn instead of every hour. On a roll of 1, the alert status returns to normal.
Some areas are always on alert.