Over and over again there are threads on here about secrecy with computers and phones. These devices used in that way are toxic to relationships. I'm a past offender. I'm working things through and one major change is that laptops, phones, iPad, etc. are now totally open all of the time. All passwords are known, nothing is erased. And frankly, that's how it should be all of the time otherwise these suspicions are bound to arise, why wouldn't they when there is no technical reason to constantly clear history.
Computers and phones keep history because doing so enhances functionality, it saves typing and helps a user recall web sites used. Forms are auto-completed so remembering passwords isn't necessary. In general, having a history speeds things up not slows things down. Any yet despite the software companies doing a great job of enhancing usability, people are keen to switch all that off! That analysis blows it all away really doesn't it.
Another reason you could express to him. When I was in max-security-lock-down mode I couldn't make available to DW things that she or the family would need in the event of an unexpected incident happening to me or both of us. She or my kids wouldn't have been able get onto the computer to find out details of bank accounts, trading accounts, investments, etc. It always niggled me at the time and now all of that is out in the open.
Re the comment about the spanner icon and other technical details, you have to remember that things are different according to the type of computer, the operating system it runs etc. The spanner is a feature of Chrome but other browsers have different icons.
If you do some searching on the Internet you could find many ways of getting into devices and monitoring usage. The trouble is you need to be a bit tech savvy and to have the time. For example, not many people realise that there's a method of booting Windows so that you can type any password you like into the logon screen and get full access. Astonishing, but true. I won't post more details in case it is seen by someone who shouldn't see it, but it's all there on the web if you look for long enough to find it.
But frankly, what you need is simply a discussion. If he knows you use MN then all you have to say is that in every case where people are so secretive there's always a bad reason, and therefore secrecy won't be tolerated.