Mod to pick up unowned items?

@KijiniiHello!  That's really strange.  I tried placing some items in world (including a treasure map) from my sim's inventory and I was able click and put in inventory.  Are you using any mods currently?  If so, maybe try moving them to your desktop then delete the localthumbcache file located in your Documents / Electronic Arts / The Sims 4 folder, then test the game to see if it's still an issue.  I was not able to find a mod that allows for picking up unowned items.

Perhaps you could try giving your sim the Kleptomaniac trait temporarily.  You'll have to use the cas.fulleditmode cheat and then take your sim to CAS to change the traits.  Then, you should be able to click an item and see "attempt to swipe" as an option.  If there are other sims around, this may fail if your sim is "caught" stealing, so try to wait and swipe the item when no other sims are around.

Otherwise, if you have Eco Living, you might try enacting the Sharing is Caring NAP.  In order for this to work, your sim has to live in the neighborhood where the item is.  You can do this with a cheat by shift+click on your mailbox and then click "Instant Enact Neighborhood Action Plan (Cheat)".  You can also do this without cheating through the regular voting process but it could take some time.  Regardless, once the NAP is enabled you should get the "swipe" option I mentioned above.  By the way, this is a Mischief action, so your sim will gain Mischief skill, and you may get a moodlet that suggests your sim has the Kleptomaniac trait (even if they don't).