Dogmeat, the main character's faithful dog companion, has a tendency to disappear. If you start wasting time trying to track him down (the settlement bell doesn't work with him), here's what you should do.
Gather the appropriate materials and build Dogmeat a dog house in one of your settlements. Build it and every time you lose him, go check there because there's a good chance that's where he'll be. (Note: There is already a dog house in Sanctuary behind one of the houses. Check there first before you check any other.)
You'll also be able to find him there if you decide to send him back to a settlement for whatever reason. This is going to help save you tons of time.