Suppose an arcane caster is carrying a shield as part of a disguise. The wizard is not proficient with the shield and is just carrying it in one hand. If the wizard decides to not use the shield, would they still suffer from the negative on the attack roll just by having it in hand but not using it?
In Pathfinder, shields have to be strapped to your arm to be considered worn, and can be carried in your hand without being worn.
In the real world, some shields are used by helding them by a handle, rather than wearing straps to support their weight.
The buckler, for instance, was hystorically held afar from your body, trying to meet your opponent's sword, while in patfinder it is strapped to your arm and leaves your hand free unlike all other shields.
So, helding a shield without wearing it is going to be extremely suspicious in the PF universe, at this point it would be equally good for your disgiuse to just carry it on your back, where it does not interfere with your attacks or initiative at all.
I'd say the Shield would have to be equipped for your example. This means, regardless of if it's practical to do so, you are using the Shield, and therefore have the negatives associated with it. The only exception is if you can come up with some convoluted explanation for how it is being visibly held but not equipped without giving away the fact you are not who you claim to be... So, you're back to having it equipped. Just drop the Shield at the first chance you get, and you should be fine. If you want, tell your DM in advance that you're readying to drop the Shield the moment somebody seems to uncover your charade. The problem is, I can't think of where it actually says anything about this specific type of thing, but I am pretty sure that is because what you're describing is meant to be, in essence, "Equip".