This topic appears fairly regularly here.
Here’s a little primer on how it works:
- The “default” intention is that the priest intends to consecrate whatever elements (bread/wine)* are on the corporal.
If the hosts are on the corporal, then the priest does intend to consecrate them, unless he has a specific intention to the contrary.
Whatever is not on the corporal is not consecrated (see below).
Whatever is on the corporal is consecrated, even if the priest is unaware that the host is there (unlikely, but possible).
- The priest may consecrate what is not-on-the-corporal, under the condition that he must actively intend to consecrate it.
Further: if the priest places the bread on the altar (but not the corporal) at the offertory, then he has such an intention. This also applies to any other situation where the priest knows that the hosts are on the altar and does intend to consecrate them—regardless of how or when they were placed there.
That paragraph applies to the OP’s situation.
If the hosts are not on the corporal, then the priest must have the intention to consecrate them.
If there are hosts outside the corporal, but the priest is unaware, then those hosts are not consecrated.
- The priest may consecrate what is not on the altar, but it must be near the altar (ie close-by the altar, not in the sacristy or some other place). In this case, he must very specifically intend to consecrate those hosts.
Some practical examples of when the hosts are not consecrated:
If a host is placed on the altar (not on the corporal) without the knowledge of the celebrant. This might happen by accident, or someone else might place it/them there intentionally. If the priest is unaware that the host is there, then it is not consecrated.
Hosts which are present elsewhere in the sanctuary, but not on the altar are never consecrated, but see #3 for the exception.
The priest can always have the intention to not-consecrate any particular hosts. For example, if the priest notices that a few stray hosts were dropped onto the altar (even onto the corporal) before Mass, but too late to pick them up (unlikely but possible) then he can intend to not-consecrate them.
At large Masses (such as papal Masses attended by hundreds of thousands) all the chalices and ciboria are placed in front of at least one priest who is concelebrating. The main celebrant (eg pope) intends to consecrate whatever is laid out for that purpose and the concelebrating priests certainly have the outward intention to consecrate whatever is immediately in front of them.
*for brevity, I’ll simply write “bread” but in reality it applies to “bread and/or wine”