Sometimes their random movements will take them farther from their patrol area, and you can repeatedly cut off their escape to move them farther and farther away. Although, this is extremely slow, it only works if the environment is tight enough for you to cut the grunt off, and it's probably not doable if you don't have big moveable barriers like warthogs.

I did this to get an active grunt in the SC security cutscene, had to use one of them from upstairs:

https://www.youtube.com/watch?v=6gd_YBt0jvk&t=1m59s