While the specific question appears to be answered, I would like to add a more general one: 

The study of the Mertens function $M(x)= \sum_{n \le x} \mu(n)$ is a notorius problem, and (thus) any new insight based on numerical investigations *for very small values* (in this context) seems most unlikely.

The function $M(x)$ is in a very vague sense about square-root-ish and thus one sometimes writes $q(x)=M(x)/\sqrt{x}$. 

On the one hand,  there used to be an old conjecture that $|q(x)|\lt 1$ (Mertens' conjecture), which was refuted by Odlyzko and te Riele (1985), and was already considered very unlikely to be true before, and Mark Lewko mentioned the curent 'record' constants. But it is beleived that $q(x)$ is in fact *unbounded*. 

On the other hand, an estimate $q(x)= O(x^{\varepsilon})$ for every $\varepsilon > 0$ is equivalent to the Riemann Hypothesis. 
More precisely, [by a recent result of Soundararajan][1] it is known that *under RH* one has 
$$
q(x)= O(\exp( \sqrt{\log x} (\log \log x)^{14})),
$$ 
and [Balazard and de Roton][2] showed that $14$ can be optimized to $5/2 + \varepsilon$ for every $\varepsilon > 0$.

Yet, it seems to be not clear (even conjecturally) how $q(x)$ should actually behave. 

[Kotnik and van de Lune (Exp. Math. 13.4)][3] made the conjecture that 
$$
q(x)= \Omega_{\pm}(\sqrt{ \log \log \log x}),
$$ 
and Kotnik and te Riele (mentioned in Mark Lewko's answer) discuss that extremal observed values of $q(x)$ are close to $\pm \frac{1}{2}(\sqrt{ \log \log \log x})$.


However, and as mentioned there, if this would remain (about) true 'forever' this would contradict  [Ng (2004)][4] [and Gonek (unpublished)] conjecture that limes superior and limes inferior of 
$$
\frac{q(x)}{( \log \log \log x)^{5/4}}
$$ 
are in fact $\pm B$ for some positive and finite $B$.  

Yet, in the in the 70's still other conjectures were made namely that the limit of 
$$
\frac{|q(x)|}{\sqrt{ \log \log x}}
$$ 
should exist, and even two(!) values were suggested for the limit. [Note: there are only two log's here.]  

And, there would still be different contributions to this; but in part I do not have access to them at the moment and also I do not really have an overview. 

In any case, this problem is complicated in that even detailed and recent investigations can arrive at different conclusions what should or might be the right expectation regarding the behavior of $M(x)$.    

  


  [1]: http://arxiv.org/abs/0705.0723
  [2]: http://arxiv.org/abs/0810.3587
  [3]: http://www.emis.de/journals/EM/expmath/volumes/13/13.html
  [4]: http://www.cs.uleth.ca/~nathanng/RESEARCH/mobius2b.pdf