R devel - Sep 2014 - "no visible global function definition" using R-Devel check utility

Hello All

If I check the package fAssets using R-3.1.1 (2014-07-10 under Windows) I
don't get any notes. If I do the same using R-Devel (2014-09-17 r66626 under
Windows) I get the following note:

.
assetsMeanCov: no visible binding for global variable 'scaleTau2'
.

I'll try to explain why with a minimal example. Let's have a look at the
following function:

assetsMeanCov <- function(x, method = c("cov", "MCD",
"OGK"),  alpha = 1/2,
sigmamu = scaleTau2, ...) {
   if (method == "cov") {cov(x, ...)}
   if (method == " MCD") {robustbase::covMcd(x, alpha = alpha, ...)}
   if (method == " OGK") {robustbase::covOGK(x, sigmamu
substitute(sigmamu), ...)}
}

where robustbase is mentioned under "Suggests:" within the DESCRIPTION
file.
This since the function is also usable without having that package
available. R check is now complaining since it cannot find the function
'scaleTau2' defined within the packages namespace or any imported
namespaces. Which is true; since this function is defined within the
robustbase package whose namespace is not imported. Note that:

1) The function works like that if the package is built. No error or warning
if method="OGK" is used.
2) By "promoting" the package robustbase to "Imports:"
within the
DESCRIPTION file and importing that function within the NAMESPACE file the
note would be gone.
3) Using "sigmamu = robustbase::scaleTau2" would solve the
"problem" as
well.

I am not very much in favour to import the robustbase package since it is
not necessarily needed. Also I don't know if using the "::" 
operator within
the functions arguments list is a good concept. Also since the code works
without having that. Therefore I am wondering if this new behaviour within
R-Devel (compared to R-3.1.1) is intentionally and if yes; what would be
best practice for such a setup?

Many thanks for your help and time!!
Tobias
> sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)