I've recently been playing with a Soldier Heavy brain. Sometimes, when I leave him alone, he just flies to a random point on the map, even when in Sentry mode. What is happening?

It's not because he's a brain, it's just the way the AI works. For some reason they favour (Generally wide open, undefendable) areas for whatever reason.

Point is, the AI sucks.

No, I noticed this a lot too in B23. Brain bots of all types would wander off on their own and do their own thing, whereas normal actors would sit still in their spot until told otherwise or attacked.

I haven't noticed this yet in B24, but that's probably because I don't use brainbots and instead only use modded actors cause the default jetpacks are worthless.

If you set an AI go-to point for it and let it go there, it should fix itself.