In this photo from Google Street View of TC's garages, I don't see a keypad to the left of the left-most garage door, but there could be one in the middle or on the right.
That said, there were mentions of relatives with spare keys. I suspect that people having spare keys could have had something to do with Bo. It sounds like TC made several trips in relation to his work. During those trips, he could've put Bo in a kennel or could've asked a relative to board him. But he also could have had someone stop by every day and walk the dog or even had someone house-sit. I haven't seen mention of relatives having a key code, though that's not necessarily something the family might want to broadcast.
Whatever happened, it's possible that there was some other way of getting back into the house besides the main set of keys, whether a keypad, a spare key that TC might've carried, or a key hidden somewhere outside.