First, if a host is seen on a shelf in a sacristy, it is certainly not consecrated. Consecrated hosts are reserved in the tabernacle. Every priest I know takes great pains to keep consecrated and unconsecrated hosts separated, for reasons that ought to be obvious.
However, if this host were consecrated, then we do have a grave matter here. Because the purpose was private adoration and not sacrilege, the latae sententiae excommunication does not apply. However, my understanding is that the Blessed Sacrament may be adored only in places approved by the ordinary bishop, and if one wishes for a very serious and compelling reason to reserve the Blessed Sacrament in his home, he must make his case for that before the bishop or one to whom he has delegated the authority. Just because one wants to is not a compelling reason.
The person here in question obviously needs to bring this to the keys, preferably through his parish priest. If the parish priest thinks this might be something grave enough, he will refer the matter to the bishop (or his canon penitentiary or like official dealing with judicial matters reserved to the bishop). I seriously doubt that this host was consecrated and that sin was incurred by action, but sin may have been incurred by will, though mitigated by ignorance. It’s a matter for the Church to judge.