I couldn't google this one either. Not a problem for me though, I've never had an issue with weapon auto-switching in the first place. When I'm not sure the area is clear, I usually switch to Cloak before doing something potentially distracting like picking stuff up, checking the map, rearranging weapon attachments etc., so that no sneaky Korean gets a drop on me. Besides, even enemies you haven't tagged with the visor still show up on your radar when they get too close, which makes it even more difficult for them to take you by surprise.

In short, just use cloak and keep an eye on the radar, and you're good.

Hey, good news!

I just made a function with Flowgraph tool in the Crysis/Sandbox Editor, that blocks the auto switching after a new weapon is picked up. I'm gonna include this feature with an update for my mod > Crysis:REDUX. This will work simply by tapping the "Interact/Use" key, and your current item/weapon held will remain selected, but if you still wish to use the old method, then holding down the key while the pickup animation happens will do this.

Now you get to have a choice in the matter! :)

https://youtu.be/TQxmIznXUI0