Why were the McCanns made by the Portuguese Police "official suspects" of involvement in the disappearance of Madeleine McCann ?
Eddie's finding in the 5A was no evidence (no sample could be taken neither in the bedroom corner nor on the balcony) but was intelligence about what could have happened to Madeleine McCann. We're in the beginning of August.
Instead of contemplating the hypothesis Madeleine was killed (possibly accidentally) in the flat and taken away by a frightened perpetrator, the McCanns insisted she had been taken alive (for reasons that are another matter). Obviously such unfounded attitude (the sniffer dogs failed to find an (alive) trail corroborating the path taken by faceless man when he was carrying Madeleine) forced the PJ team to interview the McCanns with the perspective of their possible involvement.
This happened in the middle of August and is off the record, likely because, as they weren't arguidos but witnesses, there was a legal risk their answers could be used against them. The McCanns likely failed to bring arguments against the involvement hypothesis.
After that episode they knew their one and only theory, the abduction from bed, was if not discarded by the PJ, at least had been altered. They knew also the process was irreversible. And in fact, 3 weeks later, they were made arguidos, an initiative that had a negative effect in terms of image, a crucial detail for Mrs McCann in particular, but the positive effect to allow them to participate somehow in the investigation, an opportunity they didn't seize though that would have brushed their image.