So on the find familiar spell theres an option to dismiss your familiar into a pocket dimension, does anything its carrying also go to that pocket dimension? For example say i steal a small trinket like a key or some documents or i have something small that i wouldn't want to get caught with, could i pass it to my owl friend and dismiss him into the pocket dimension to hide the goods?

The text says you dismiss the familiar and has no mention about anything the familiar might be carrying. Perhaps this means only the familiar goes and anything it is carrying just drops to the floor. This is the more direct intepretation.

However, that is boring isn't it? I mean, what if you had some cool threads for your familiar? A collar for the cat, a ribbon on the raven, (yay alliteration) or something. You'd have to pick these up and add them back each time. Dull. So, the more fun ruling would be to say "yes" as long as it was small enough to be carried or worn without causing encumberance sure.

But there's a problem. If you die with your familiar bamfed that item is sealed away beyond any means of retrieval forever because your death actually does not affect the familiar - it remains as is either forever or until you return. Death does not break the familiar's service. It cannot be dispelled either. Being able to have your familiar secure an item beyond the reach of any magic, access or methods of retrieval by any means by any being in all of existence forever is a seriously powerful utility for a Level 1 spell. You would basically be getting free use of Nondetection and a smaller but otherwise superior version of Demiplane. It's a bit much.

In my game if I allowed this feature I would homebrew some changes to how find familiar works: if somebody was using divination to locate the object while it was in the pocket dimension I would have it point to you, if you died your familiar "dies" (as if dropping to 0 HP) but remains bound - the items drop to the floor and if it was in the pocket dimension then they appear in an unoccupied space near you (if you return to life you can cast find familiar to bring your familiar back as normal). That way if the enemy really wants the item they have to kill you to get it. Therefore it is a very useful utility but to secure something for the true long term you would need to find other methods.

With those homebrew rules in place I would allow it and you're still getting a wonderful feature. You could, for instance, dismiss your familiar and look through a window into a shop and summon them inside (30 ft, space you can see). They can pick up an item and you dismiss (no limit, can do anytime) and summon them back next to you. You could now steal things through barriers like windows, gates, etc and without using spell slots. As if familiars weren't super useful enough.

Sounds like fair Homebrew to me