I couldn't find any evidence for it having sedative properties in humans. Like I said it's not listed as a side effect. Haven't noticed that side myself either.
Guaifenisen is one of those strange drugs.... Its commonly used for one purpose in veterinary medicine across multiple species, but you have a hard time finding a reference to it having the same use in human medicine. There are limited references since the 1960's using it as a sedative type drug in humans. I think for the simple fact that it really has no analgesic properties. If you compare human and veterinary anesthesia, human anesthesia has a very strong curve towards drugs and combinations that provide at least some form of analgesia. In veterinary medicine, thats often overlooked and honestly, is only recently being considered. Traditionally the approach was to "keep em down". There are still heated debate to the degree of pain some species "feels".
Guaifenisen is commonly combined with opoids also in human medicine. Sedation is listed as a side effect of these combination drugs, but its not specified if its due to the guaifenisen or the opoid.
This drug is used too often for its sedative/muscle relaxing in too many species in veterinary medicine for me to think that it doesn't have a similar effect on humans.