I am yet to completely confirm this. But I'm very confident saying that in this situation this noise is due to low fluid level on the transmission. It was pointed out by someone on volvoforumsuk that it could be the reverse idle shaft making the noise, because it was above the fluid level (or rather the fluid was bellow).
THis was made worse by the fact that I?ve been parking the car downhill recently, so the already low fluid level was pushed to the front of the transmission.
This theory is further validated by both the fact that after driving for a while the car would stop making the noise (because it got lubricated during driving) and by me changing the direction of the car at the parking spot, meaning facing up, forcing the oil to the rear and therefore submerging the rev. idle shaft which caused the car to not make this noise anymore.

I'm yet to check the level because I'll just order the oil first. But I'm pretty sure that's it. Quite a weird one I guess.

So if anyone has a weird accute squeaking noise only in reverse driving uphill that goes away after driving a bit, check the tranny fluid level.