New member here! And an Aberdonian living in the city.
I've previously worked in and around the area the sisters went missing and I would say I am fairly familiar with the comings and goings in that area. The road adjacent to the river path (South Esplanade West) is notorious for drug deals and shifty vehicles hanging about at night. It's also a perfect slip road in and out of the south side of the city. The commercial premises along that way also have their fair share of break-ins. It's not somewhere you want to go walking at night and EVERYONE knows that. It's dark and uninviting. The sisters certainly did not (IMO) walk that way whilst on a leisurely stroll. It makes absolutely no sense. In my mind they took that route because it is out of sight and they did not want to be seen, for whatever reason (to meet someone, to take their lives, to hide from someone). One thing I'm unsure of is whether there are any holes in the boundary fence separating the river path from the back of the commercial premises. There are two buildings/areas on that stretch of road I think they could have quite easily jumped into a vehicle without being seen if they were able to get through gaps in the fence. That road and that area is well known by police to be dodgy.
I'm intrigued by the "items/materials" found in the flat that raised immediate concern. Strange terminology used by the presser. "Materials" makes me think of drug making/taking paraphernalia or documentation of some sort, but it could just be an awkward way of referring to clothing. I don't think their intention to leave the property is a typical sign of intending to take their own lives. If it was was their intention to take their own lives I could understand them leaving documentation laid out to be found in the flat, but surely by telling their landlord they intended to leave it would have drawn attention to something they were trying to hide (just MO).
They appeared to be dressed quite smartly on the night of their disappearance which could suggest they had been out socialising, and might also explain why they were walking at 2am (although it DOESN'T explain why they were walking in that location). If they were living a life unknown to their family they could well have been meeting someone there to buy drugs or to sell some part of themselves (sorry). The CCTV image from Charlotte Street seems to show them walking with intent and purpose, however there's no time stamp on the image so we can't be sure when it was recorded. I note they both have their hair tied up in a bun/band - I wonder if anyone who knows them might be able to suggest if that has any significance (dancing, working).
The sad thing about cases like this is they could have been extremely depressed and hidden it from everyone. If they did make a plan it's very likely only the two of them knew what that plan was.