Please help us help you and fill where relevant:
Your LimeSurvey version: 5.6.9
Own server or LimeSurvey hosting: own server
Survey theme/template: Fruity
==================
Hello,

I would like to collect the data from Browser Detect (new in 5.x), ie: "the browser the respondent is using and the operating system" but without making it apparent to the rispondant (the person is informed about that, but I don't want to ask a question about it"

When I let the question visible, it appears on the questionnaire and the data are saved. When I click on "always hide the question", the data are not collected in background
Is it a way to collect the browser and the OS in background?

Thanks in advance for the help! 

Give the question the CSS class .hidden in the advanced settings.

If you use "always hide" the question is not rendered and not available in the HTML source code, so Javascript can't write to a question, that doesn't exist for it.

In a question theme like that, it should probably be a question setting to hide it with CSS.

Having said that, I find the question theme a little rough anyway. Why does it have settings like location, prefix/suffix, timer...?

In a question theme like that, it should probably be a question setting to hide it with CSS.

Agreed. Some of the new features come quite "raw".

gitlab.com/SondagesPro/QuestionSettingsT...indUserAgentInfo.php

Do it on hidden question (using prefilling value)