Meh, in my view, when the auditor has left site, I wouldn't battle on a single non con unless it fundamentally changed a customer's view of me.
What if you mentally flipped it on its head and said "ok, perhaps a single ant WAS a sign of something, what could it be and what do we have in place to mitigate and prevent?"
Do an RCA and include all of the controls you have or could have. Are they working? For example look for:
Are pallets stored outside?
What is door control like?
Are there any gaps in fabrication?
Do you have crawling insect monitors? Are they correctly sited and are they showing activity?
What kind of ant was it? Garden and casual intruder? Something harder to get rid of e.g. Pharaoh ant?
How do you train staff in pest awareness and reporting? Is that effective? I.e. do you have any reports from your teams?
How are your pest actions followed up on? Are they up to date?
How are pests avoided off shoes? Do you have captive footwear or mats? Are they working?
I'd then compile all of that and more I think you will think of specific to site. Do any actions, write it up, close out non con.
In my view, is it picky? Hell yes but one ant in an area rarely in my experience actually means one ant. It means there are probably more. I'd want to rule that out and if nothing else it's a good and thorough review of your crawling insect (intruder and SPI) controls. You might find other things when you really delve. I'm not sure if they service US as well but we used to have a company called Acheta in the UK, I think now owned by Kiwa who do in depth and response visits. Not crazy expensive and really look at what your pest management companies miss. Trust me, they miss A LOT. All of them. Your technologists start doing short cuts, get to know the site too well etc. If you get the chance to get someone like that in at some point, I would whether it's for this issue or just in general. For most pest management companies if they sent me a letter head with "this isn't an issue" I'd think BS. I've found too many incidents of pest activity in my own sites and supplier sites to believe that. So I think that would go down (in this glorious British phrase) like a bucket of cold sick.