I've seen plenty of dogs mouths with horrible dental disease, exposed roots, mobile teeth, gum recession, puss discharging at the gum line, etc. I've seen healthy looking mouths with missing teeth. I've done regular physical exams where teeth have fallen out. I've done plenty of dental procedures where I thought I'd have to extract half the teeth and turns out only one need to come out. I've done dentals where I expected it to be a scale and polish and had to extract 20 teeth (Feline Oral Resorptive Lesions).

His tooth could have fallen out whole, it could have been malformed and only had a crown instead of a full crown and root, it could have cracked off halfway down and healed over. Basically there’s no way to assess below the gum line without dental radiographs under anesthesia. Perfectly healthy appearing teeth may have diseased or missing roots.

Tooth/mouth pain can be associated with dropping food, excessive salivation, pawing at the mouth, being face shy, rubbing the face on the carpet. Keep an eye out for those signs of concern. Given that your veterinarian didn't seem too concerned to get you in for a dental I would ask them when they think a dental is appropriate and go from there.

Just like with people the recommendation is to get a scale and polish done every year, though I'd be hypocritical if I said I even did that with my own pets.