You don't have to tell them anything. An ERPO is a court order prohibiting you from possessing guns and a warrant allowing them to take them from your home. It does not independently impose any duty on you to tell them the location of all guns that you own. Such a duty could be unconstitutional anyway (if you have AWs under the auspices of MH's dictates, non-FA-10'd guns, anything within gray areas of MA law, etc, that's self-incrimination).
They come to your home. Provide a vague but plausible explanation for why what's in your safe is all you own despite the delta between what's in their and their FA-10 list. It's not like they're going to put you under surveillance. This isn't a federal investigation or even a criminal one.
In all likelihood they won't be particularly thrilled about having to deal with serving this BS anyway, especially if, as I expect, we get people trying to weaponize this and go to the cops to get a suitability revocation, and when the PD calls BS on them they get an ERPO from an MA marsupial judge anyway.