I know of no specific requirements for grounding an aircraft for "storage" within a hangar. However, there are mandatory requirements that address electrical system grounding, and static grounding. These are two separate issues. In the first, the National Electrical Code (NEC), Article 513.10 Special Equipment, (A), Aircraft Electrical Systems,(C), External Power Systems for Energizing Aircraft, (3), Cords; require that the Flexible cords used to power the aircraft "shall include an equipment grounding conductor". DC power system pendant cord plugs contain two contact pins, a + V and - V, the - V conductor is usually grounded at the power source; but this does not satisfy the Article 513 requirement above. In the case of 3-phase 400-HZ connectors, 6 active pins are provided in the standard connector; Pins for phases A, B, C, and the N neutral, as well as an "E" and "F" interlock circuit. The "N: Neutral pin must be bonded to building steel at the source. As noted above, the aircraft structure is used as a "return" path for both DC, and AC returns.
Static grounding can be satisfied by the use of a static grounding strap. This is mandatory for US Military aircraft. I have no data on commercial or private manufacturer's requirements.
In the past, there have been horrific hangar fires, such as the Sabena Hangar fire in Belgium in May of 2006. Issues that need to be addressed are: compliance with NEC, effects or multiple voltage systems attached to parked aircraft.
Before I retired, I addressed these issues in an article for EC&M Magazine; see at https://www.ecmweb.com/design/electrical-equipment-issues-parked-aircraft