Imagine keeping a briefcase full of cash at a loyal friend's house, then he told you his crack smoking father just moved back in claiming he's man of the house and all that.

I'd get my shit before it gets confiscated

I think you're right, but in diplomacy the real reason doesn't matter. It's like a woman saying they have to powder their nose when she in fact is holding in a big fart. If the Dutch would be honest about their reasons, they would upset the orange toddler, which would not bring them any good.

Trump ordered an audit of the gold stores at Fort Knox. If he just seized all the Dutch gold, what could they do about it?

They just sold most of it and purchased gold in London. The remaining 27 metric tonnes they did ship would only take up 1.4 cubic meters as a solid block, so probably 2-3 cubic meters as bars. A large shipping container can hold about 68 cubic meters, though they'd likely have to spread the gold out to better distribute its weight no matter the shipping method.

France has basically all of its gold reserves in Paris, since Charles de Gaulle moved them there. Germany has 37% of its gold reserves in the US and there is talk about bringing them back. Italy is at 43% and there seems to be basically nothing, but some discussion in some newspapers.

https://www.ft.com/content/e39390cc-ea02-4197-843a-1e4c242422cc?syn-25a6b1a6=1

One thing which is really important here is that the Dutch just sold their gold in New York and at the same time bough gold in London. They did not ship it at all. For Germany and Italy that is not an option, as the gold reserves in the US are so massive, that it would destroy the market. Both combined have about 1% of the gold in the world sitting in the US. Obviously a lot less is physically traded.

EDIT: Also this has to be done fairly quickly. As to not spook the market and piss of Trump.