The caveat is 'if the mystery couple exists'.
If they exist they may have come forward to Police and have already been eliminated from any involvement in the subsequent disappearance - the statements that refer to the 'mystery couple' are not in the public domain AFAIK.
One possible scenario is if the 'mystery couple' do exist and did enter the apartment, then I would suspect they were either contacted by Mrs Fenn or they are another resident of the block.
It would be fairly unusual to enter an apartment in these circumstances, but if they were aware that the parents left the children unattended and knew the child had been crying for a long time, someone might decide to make a check.
I would have gone through proper channels, but someone may have not have wanted to involve the Police, or there may be other reasons, such as they felt it was an emergency situation.
http://www.express.co.uk/news/uk/400796/Mystery-couple-seen-going-into-McCanns-flat-on-night-before-sobbing-Madeleine-disappearedSCOTLAND YARD detectives are trying to find a middle-aged couple said to have entered Madeleine McCann’s holiday apartment to comfort her because she was crying, we can reveal today......
The tip-off was given by two key witnesses who were reinterviewed as part of the Yard’s two-year, £4.5million investigation........
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From that article we can glean there are definitely two separate couples, one known to SY following interviews prior to May 2013 (so presumably a UK couple) & another unknown couple seen by the aforementioned entering 5A.
From that we can deduct that the known couple either
a) totally withheld from the original investigation all information about what they saw
b) Told LP who failed to forward the info to the PJ or
c) Told LP who sent details to the PJ which was then ignored.
Unless Madeleine had ventured outside, imo it is beyond belief that anyone would enter someone else's apartment simply because a child could be heard crying and furthermore leave the children alone again without attempting to trace the parents. Would Madeleine really have just settled back down to sleep in the presence of 2 strangers? Totally ridiculous imo.
Mrs Fenn stated that she was out on the Wednesday evening. Rachel was next door, unwell. The Moyes couple arrived on the Wednesday evening - I will have to check to see if there is any record of the arrival time.
Is it possible that the known couple only actually saw the Moyses going into Block 5 & the story has been somewhat embellished after source?