This is an irregular verb. It follows the conjugation pattern of the verb subir.
This verb is more common in Brazilian Portuguese. In European Portuguese, desaparecer is preferred.
Present:
As chaves somem o tempo todo.
The keys disappear all the time.
Preterite:
O Miguel sumiu depois da reunião.
Miguel disappeared after the meeting.
Conditional:
A Clara sumiria sem avisar.
Clara would disappear without warning.