The $2/$5 rock who calls preflop from the blinds and then check-calls two streets is almost certainly on a draw or has a hand they're not sure about. When the draw misses and they check the river, they're done.

Why they do it

$2/$5 rocks are passive by nature. They're not trapping with check-calls - they're confused or chasing. When the river blanks and they check, it's not an invitation to bluff-catch. They're giving up.

The adjustment

Bet the river when the draw misses. A small to medium bet gets folds almost every time. They weren't calling you down - they were hoping to get there.

What it's worth

Checking back a river against a rock who was on a missed draw leaves $40-80 on the table. One river bet here per session is found money.