I added a filter to an incoming email and it said "Apply to 79 other messages". I didn't see where these other messages existed. After applying the label, I now see these messages in that label.
I am trying to figure out why these emails didn't show up in my inbox. Is there anyway to see where these came from and how to have them show up in my inbox?
If those emails weren't in your Inbox then they must have previously been archived. (Archiving simply removes the "Inbox" label.)
Archived emails are still available under "All Mail" (label/option on the left). However, the "All Mail" option is not shown by default. To show it, go to Settings > Labels ... System Labels > All Mail > show.
If they are in your Gmail account they were at one time in your inbox, unless:
Note that a message/conversation can have multiple labels. Your new filter just added a label to those messages. The filter can archive a message by removing the inbox label, but it didn't remove any other labels already attached to the message.
Once you have moved an email or set of emails using a filter in GMail, there is no way to tell where they where stored before.
However, when you are on the filter creation screen, all of the emails you are about to modify are listed below the options area with a text indication of where they are currently stored.
If you must figure out where a similar email would be sent you could first disable the filter temporarily and attempt to send a copy of one of the emails in question to yourself using an email spoofer (to also have it appear to arrive from the same source as the original - Google for "email spoofer" returns many results).
It is very possible that they were simply being spam filtered to begin with.
