If you can't see a tick it might be a sebaceous gland that is clogged (a bit like a pimple) or it might be a scab from a tick bite or a small infection. Despite all lore to the contrary ticks only bury their heads in the skin. It might be the dog knocked the body off and left the head embedded though.
You can see if using some neosporin or triple antibiotic ointment on it makes it go away.
If not then you should have your vet check it out. I'd put on ointment twice a day for a day or so and see if the spot improves.
Hope this helps you!
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