Some more thoughts, as I cannot stop thinking about this...
I found the other threads (must have been blind before) and saw some info about the shirt has been mentioned, but no followups on it. So, still interesting.
I'm not so sure about the Canada connection. It comes from
one person, months after the murders happened. He could have had reason to lie, he could have remembered somebody else, and even if he was correct, Jock/Jacques could have lied. (Of course, that uncertainty is particularly frustrating, as it is the only real clue about this person.)
Something about him sounds European, I think. The Lady, not so much.
Unfortunately, there is no combined European database for missing persons on the net. When I first found NAMUS some months ago, I asked a former german police officer why there is nothing like it over here. He said in his active days (which would be the 70ies and 80ies, he moved on to other duties later), adult persons were only registered missing by police if they had "serious reason to believe there could be a crime". So, if someone just walked out of his home never to return, or did not come back from a holiday, there probably wouldn't even be a file on this person. (This could be different in other countries.)
So, the couple could well be missed, but their relatives / friends turned down by police, and didn't know where to start. Or searched on their own but lost hope and faith at some point in the past.
I've searched for forums like this one in the languages I know (german, french, swedish) but didn't find much. Only cases that are well known at least in the area where they happened. Most archives don't go back further than the year 2000.
But even if someone was on file as missing back in the 70ies, it wouldn't be easy to find out about that. Most local law enforcement websites don't have data from pre-internet days available, and many places do not have any records online at all.
This post is getting way too long without actual substance, sorry about that. Thoughts are roaming.
About the car: To me it seems most probable that the car (van) that was heard that night and left the tire prints belonged to the couple. There is this mechanic witness who thinks has seen him, plus nothing about them says "hitchhikers". And it would solve the riddle about the missing luggage, if the suitcase was in the van and stayed there. I think the killer just drove off with the car and disposed of it later. Or it may even be still around.
This is again a piece from my personal memory - back in the days, the idea was very popular to fly to the states (east coast), buy a car, do a slow coast to coast drive with sightseeing along the way and then sell the car at the west coast before flying home. I don't know how many people actually did this, but even if it was only 1% of the people talking about it, that would be enough for a thriving business specializing in selling cars to visitors. Does anyone know of businesses like this?
And a question - has anybody ever done an overview map of the places they were known to be and the places they could have been? Murder Site, camp ground, the place where the mechanic believes to have seen them - plus possible race tracks, the truck stops from the matches, the marina that was talked about? Not sure if it would help, but it might.
OK, I'll shut up now.