There is two bugs with assets:
- Assets in/on destroyed items are included (weapons etc. on destroyed ships and items in destroyed containers): https://github.com/ccpgames/esi-issues/issues/698
- Assets with locationID above 9000000000000000000: https://github.com/ccpgames/esi-issues/issues/684 (I’m not sure if this have been fixed, yet. One of the devs messaged me about it and they will be/have been removed from the assets results, as they’re destroyed items).
EDIT: The full list of known and unknown locations: Asset location_id quick reference