“All the world's a stage, and all the men and women merely players”
I have conducted usability testing in a number of environments both controlled and uncontrolled and I genuinely believe that there is no silver bullet to deal with bias, but you do have to take precautions and consider impact on test results.
Erving Goffman who was a very influential sociologist wrote a book in the 60s entitled The Presentation of Self in Everyday Life in which he describes the theatrical performances that take place in face-to-face interactions. Sorry for the long quote but it is so relevant:
When an individual comes in contact with another person, he attempts
to control or guide the impression that the other person will form of
him, by altering his own setting, appearance and manner. At the same
time, the person that the individual is interacting with attempts to
form an impression of, and obtain information about, the individual …
participants in social interactions engage in certain practices to
avoid embarrassing themselves or others. Society is not homogeneous;
we must act differently in different settings. This recognition led
Goffman to his dramaturgical analysis. He saw a connection between the
kinds of "acts" that people put on in their daily lives and theatrical
performances. In a social interaction, as in a theatrical performance,
there is an onstage area where actors (individuals) appear before the
audience; this is where positive self-concepts and desired impressions
are offered. But there is, as well, a backstage – a hidden, private
area where individuals can be themselves and drop their societal roles
and identities.
Taking precautions to minimize onstage performance:
Every testing situation is unique and depends on who you are testing with but as a rule you need to account for situations that are more likely to have an impact on "onstage performance" for example by:
Preparing test participants and reassuring them: This allows you to inform and reassure testing participants that the usability testing session is not meant to test their abilities but rather the product performance (should be part of a formal testing protocol that also has a confidentiality agreement).
Configuring usability testing environment: This allows you to assess the testing environment and decide on optimal layout for testing to take place. you should be able to clearly observe what is happening while gradually easing yourself out so users don't focus to much on the fact that they are being observed.
Analysis and focusing on the "backstage":
As results are bound to have some onstage issues, you need to factor those in for both yourself and test participants. The way I go about this is by constantly questioning user actions at each action point. This translates to two list, first: is "why the user is doing this" and second: "because" then i list all possible triggers along with explanations till I have exhausted all possibilities, which should in principle allow me to spot any onstage performance while zooming in on the Backstage stuff:)
Remote usability testing bias
This one is a bit tricky, I think you might be right about suspecting a bias of some form but you could mitigate this by checking and reviwing the recruitment process,demographics,type of reward,if users have participated in many remote testing sessions before or not! my last point, they are becoming professionals and thus not right for the job :)