In theory, I think claimants are supposed to notify DWP if they are going to be away from home. In practice, I probably wouldn't bother to tell them about a few days in hospital as long as there will be someone to check your post while you are away.
If you do decide to notify DWP, doing so will not trigger reassessment as the call centre workers are separate from the case managers (and reassessment is triggered automatically by computer anyway).
If you are also in receipt of DLA/PIP/AA, payments will be suspended after 28 days in hospital.
Nothing on this board constitutes legal advice - always consult a professional about specific problems