I agree, the hosts cannot be accidentally consecrated. But the tin should not have been left on the altar.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“306.
Only what is required for the celebration of the Mass may be placed on the mensa of the altar: namely, from the beginning of the celebration until the proclamation of the Gospel, the Book of the Gospels; then from the Presentation of the Gifts until the purification of the vessels, the chalice with the paten, a ciborium if necessary, and, finally, the corporal, the purificator, the pall, and the Missal. In addition, microphones that may be needed to amplify the priest’s voice should be arranged discreetly.
307. The candles, which are required at every liturgical service out of reverence and on account of the festiveness of the celebration (cf. above, no. 117), are to be appropriately placed either on or around the altar in a way suited to the design of the altar and the sanctuary so that the whole may be well balanced and not interfere with the faithful’s clear view of what takes place at the altar or what is placed on it.
308. There is also to be a cross, with the figure of Christ crucified upon it, either on the altar or near it, where it is clearly visible to the assembled congregation. It is appropriate that such a cross, which calls to mind for the faithful the saving Passion of the Lord, remain near the altar even outside of liturgical celebrations.”
