I'll go back and have another squiz just to be sure but here are my thoughts at this moment.
The key part is the layout of block 5 (yup, block 5, not block 6).
Block 5 has a single central stairwell. Every apartment above ground floor has to be accessed from this stairwell.
The four apartments on the ground floor have a pathway in front of them. This pathway is separated from the car park by a wall about 1m high. The way one gets from block 5 into the car park or from the car park into block 5 is via a gap in this wall, and the gap is directly in front of the central stairwell. So everyone in block 5 has to go very close to the central stairwell.
Unless you have a completely unhindered view of the front of block 5, you can tell people are going in and out of the car park, but you can't tell whether someone is using a ground floor entrance, or the central stairwell.
Block 6 is L shaped, with 2 stairwells. The short part of the L is opposite 5A, the long leg heads east away from block 5.
One stairwell is half-way up the long leg of the L, perhaps 40 or 50m from block 5, and there is no view whatsoever of the rear of block 5. So let's move to the other stairwell.
That is where the long leg of the L and the short leg of the L come together, fairly close to the entrance of the car park for block 6.
There are 5 apartments on the ground floor. Apartment 5 occupies the short leg of the L. Apartment 4 is the one heading eastwards from this stairwell. One floor up is apartment 8 (above 4), and apartment 9 (above 5). I presume on the final floor it is 12 (above 8) and 13 (above 9).
So 5, 9, and presumably 13, are directly opposite block 5. The rear patio of these is a great place to monitor the Tapas area and the rear of block 5. The rear patio is near useless for monitoring the front of block 5. You get to see part of the block 5 car park near its entrance if you are lucky.
From the front of this stairwell in block 6, you get a partial view of the rear of apartment 5A, but still not a clear view of the front. That pesky central entrance/exit to block 5 is the problem.
In descending order of probabilities.
Apartments 5, 9 and 13. Ability to clearly see the back from private balcony. Limited ability to see the front from public access.
Apartments 4, 8, and 12, the ones using the same stairwell. No ability to see apartment 5A from private balcony at rear. Public access at front gives a limited view of the front of 5A, and a limited view of the rear.
Anyone not connected with these apartments. Same access as 4, 8, 12 but no legitimate reason to spend hours monitoring there, thus standing out like a sore thumb.
I will pay another visit some time this week, to confirm.
The law on smoking in commercial establishments (pubs, restaurants) changed on 1/1/2008. The establishments had to choose to be non-smoking or smoking (and had to install suitable extractor fans). Of the 3 that I know remained smoking, all are owned/run by Portuguese folk.
If you have read Amaral's book, he recounts late night progress meetings where you could cut the smoke with a knife.
According to the PJ files, the Portuguese police were using one of the apartments in block 6 as a police station.
So, probabilities again, this time in ascending order.
Butts belonged to the police, so no interest.
Police could see that the location of the butts was not a place to monitor.
Butts came from someone who monitored, and is implicated.
Butts came from someone after the event, e.g. someone trying to get a different view.
Butts came from someone unconnected, probably from a country where non-smoking laws had been implemented for some time.
