This is of course second-hand information which we shouldn't necessarily assume is accurate, although the fact that it's such a mixed bag for Labour means that it has the ring of truth to it. It would also be consistent with the fact that the party do seem to be doing more than simply going through the motions in their Scottish campaigning.
However, to offer a reality check, there were similar reports in the run-up to the 2007 Holyrood election. In fact, Nick Robinson announced live on the BBC that Labour sources had told him they were ahead in seats that they were supposed to be behind in. If memory serves me right, we even saw exact figures from Ochil (in breach of the rules) showing Labour in the lead.
In the event, the national result was indeed a bit closer than the opinion polls had been suggesting, so in that sense what we're seeing tonight may be a meaningful warning sign. But on the other hand, the SNP went on to win many of the seats that Labour had been so excited about after looking at the postal votes. In Ochil, for example, the result was SNP 38.5%, Labour 36.9%. People who vote by post aren't necessarily going to be a representative cross-section of the general population - they may be older, and more small 'c' conservative. If so, it could be that the real SNP surge won't make itself felt until Thursday itself.
All the same, this is a useful reminder that the situation remains uncertain, and it's important to keep campaigning hard until the very last second.